In 1916, Romania entered World War I on the side of the Allies. In doing so, Romania's goal was to unite all the territories with a Romanian national majority into one state. In the Treaty of Bucharest (1916), terms for Romania's acquisition of territories within Austria-Hungary were stipulated.

In 1918, the Austro-Hungarian monarchy politically collapsed and disintegrated as a result of a defeat in the Italian Front (World War I). During the war Count Mihály Károlyi led a small but very active pacifist anti-war maverick fraction in the Hungarian parliament.

In 1918, after the October Revolution, the Bolsheviks signed a separate peace with the Central Powers in the Treaty of Brest-Litovsk.

The Union of Bessarabia with Romania was signed on 9 April 1918. The unification act that brought these lands within the modern Romanian state was not recognized by Bolshevik Soviet Russia, but it was occupied with fighting the White movement, Poland and the Ukraine in its war for independence, and resources were not available to challenge Romania.

Romania was alone on the Eastern Front, a situation that far surpassed its military capabilities. Therefore, on 7 May 1918, Romania sued for peace. The prime minister of Romania, Alexandru Marghiloman, signed the Treaty of Bucharest (1918) with the Central Powers. However, this treaty was never signed by King Ferdinand of Romania.

On 31 October 1918, the Aster Revolution in Budapest brought Hungarian liberal aristocrat Mihály Károlyi, a supporter of the Allied Powers, to power. The Hungarian Royal Honvéd army still had more than 1.400.000 soldiers when Mihály Károlyi was announced as prime minister of Hungary. Károlyi yielded to U.S. President Woodrow Wilson's demand for pacifism by ordering the disarmament of the Hungarian army. This happened under the direction of Béla Linder, minister of war in the Károlyi government. Due to the full disarmament of its army, Hungary remained without a national defence at a time of particular vulnerability.

On 5 November 1918, the Serbian army, with the help of the French army, crossed southern borders.

On 8 November, the Czechoslovak Army crossed the northern borders.

On 10 November 1918, taking advantage of the Central Powers' precarious situation, Romania re-entered the war on the side of the Allied forces, with similar objectives to those of 1916. King Ferdinand called for the mobilization of the Romanian army and ordered it to attack by crossing the Carpathian Mountains into Transylvania. The end of World War I that soon followed did not bring an end to fighting for the Romanian army. Its action continued into 1918 and 1919 in the Hungarian–Romanian war.

Following the 1918 Treaty of Bucharest, the bulk of the Romanian Army was demobilized. Only the 9th and 10th infantry divisions and the 1st and 2nd cavalry divisions were at full strength. However, those units were engaged in the protection of Bessarabia against the Bolshevik Soviet Russians. The 1st, 7th and 8th Vânători divisions, stationed in Moldavia, were the first units to be mobilized. The 8th was sent to Bukovina and the other two were sent to Transylvania. On 13 November, the 7th entered Transylvania at the Prisăcani River in the eastern Carpathians. The 1st then entered Transylvania at Palanca, Bacău.

On 13 November, Károlyi signed an armistice with the Allied nations in Belgrade. It limited the size of the Hungarian army to six infantry and two cavalry divisions. Demarcation lines defining the territory to remain under Hungarian control were made.

On 13 November, the Romanian army crossed the eastern borders of the Kingdom of Hungary.

The lines would apply until definitive borders could be established. Under the terms of the armistice, Serbian and French troops advanced from the south, taking control of the Banat and Croatia. Czechoslovakia took control of Upper Hungary and Carpathian Ruthenia. Romanian forces were permitted to advance to the River Maros (Mureș) . However, on 14 November, Serbia occupied Pécs.

Later, Romanian units reached the line of the Maros (Mureş) River. This was a demarcation line agreed upon by the representatives of the Allied powers and Hungary at the Armistice of Belgrade. At the same time units of the German army, under the command of Marshal August von Mackensen, retreated to the west.

On 1 December, the Union of Transylvania with Romania was officiated by the elected representatives of the Romanian people of Transylvania, who proclaimed a union with Romania. Later the Transylvanian Saxons and Banat Swabians also supported the union.

On 24 December, units of the Romanian Army entered Kolozsvár.

By 22 January 1919, the Romanian army controlled all the territory to the Maros River. The 7th and 1st divisions were spread thin, so the 2nd Division was sent to Nagyszeben and the 6th Division to Brassó (Braşov). Two new infantry divisions, the 16th and 18th, were formed from Romanian soldiers previously mobilized in the Austro-Hungarian Army. A unified command of the Romanian army in Transylvania was established.

On 28 February 1919, at the Paris Peace Conference, the council of the Allied nations notified Hungary of a new demarcation line to which the Romanian army would advance. This line coincided with railways connecting Szatmárnémeti, Nagyvárad, and Arad. However, the Romanian army was not to enter these cities.

On 19 March, Hungary received notification of the new demarcation line and demilitarized zone from French Lieutenant Colonel Fernand Vix (the "Vix note"). The Károlyi government would not accept the terms and this was a trigger for the coup d'état by Béla Kun, who formed the Hungarian Soviet Republic.

The Károlyi government failed to manage both domestic and military issues and lost popular support. On 20 March 1919, Béla Kun, who had been imprisoned in the Markó Street prison, was released.

On 21 March, Béla Kun led a successful communist coup d'état. Károlyi was deposed and arrested. Kun formed a social democratic, communist coalition government and proclaimed the Hungarian Soviet Republic. Days later the Communists purged the Social Democrats from the government.

On 21 March 1919, Romanian troops of the 39th Regiment occupied Tiraspol.

A demilitarized zone was to be created, extending from the new demarcation line to 5 kilometres (3.1 mi) beyond the line. The demilitarized zone represented the extent of Romanian territorial requests on Hungary. The retreat of the Hungarian army behind the western border of the demilitarized zone was to begin on 22 March.

On 4 April, South African General Jan Smuts was sent to Hungary. He carried the proposition that the Hungarian communist government under Kun abide by the conditions previously presented to Károlyi in the Vix note. Smuts' mission also represented official recognition of the Kun communist government by the Allied council.

When Kun became aware of Romanian preparations for an offensive, he fortified mountain passes in the territory controlled by the Hungarian Army. Then, on the night of 15–16 April, the Hungarians launched a preemptive attack.

When Kun declined the terms of the Vix note, Romania acted to enforce the new railway demarcation line. Romania planned to take an offensive action on 16 April 1919. The north battalion was to take Nagykároly and Nagyvárad. This would separate the elite Hungarian Székely division from the rest of the Hungarian army. The north battalion would then outflank the Hungarian Army. Simultaneously, the south battalion would advance to Máriaradna and Belényes.

By 18 April, the first elements of the Romanian offensive were completed and the Hungarian front was broken.

On 19 April, Romanian forces took Nagykároly.

On 20 April they took Nagyvárad (Oradea) and Nagyszalonta (Salonta). Rather than following the instructions of the Vix note, the Romanian army pressed on for the Tisza River, an easily defended natural military obstacle.

On 23 April, Debrecen was occupied by Romanian forces.

The Romanian Army began preparations for an assault on Békéscsaba. On 25–26 April, after some heavy fighting, Békéscsaba fell to Romanian forces.

The Hungarians retreated to Szolnok and from there across the Tisza River. They established two concentric defense lines extending from the Tisza River around Szolnok. Between 29 April and 1 May, the Romanian Army broke through these lines.

On 30 April, French Foreign Minister Stéphen Pichon summoned Ion I.C. Brătianu, the Romanian representative to the Paris Peace Conference. Romania was told to cease its advance at the Tisza River and retreat to the first demarcation line imposed by the Allied council. Brătianu promised that Romanian troops would not cross the Tisza River.

On 1 May, Bolshevik Soviet Russian Foreign Minister Georgy Chicherin issued an ultimatum to the Romanian government. Romania was ordered to leave Bessarabia.

On the evening of 1 May the entire east bank of the Tisza River was under the control of the Romanian army.

On 2 May, Hungary sued for peace via a request delivered by his representative, Lieutenant Colonel Henrik Werth. Kun was prepared to recognize all of Romania's territorial demands; requested the cessation of hostilities; and asked for ongoing control of Hungarian internal affairs.

Under the command of Vladimir Antonov-Ovseyenko, Bolshevik Soviet Russian troops gathered along the Dniester River in preparation for a large attack on Bessarabia on 10 May.

With cessation of hostilities, Kun worked to improve his battered international position. On 20 May 1919, a force under Colonel Aurél Stromfeld attacked and routed Czech troops from Miskolc.

Bolshevik Soviet Russian attacks in Bessarabia intensified, peaking on 27–28 May with an action at Tighina.

The Romanian Army's 4th and 5th infantry divisions were moved to Bessarabia. In southern Bessarabia a territorial command unit formed by the Romanian Army's 15th Infantry Division was established. By the end of June tensions in the area had eased.

On 3 June, Romania was forced into further retreat but extended its line of defence along the Tisza River and reinforced its position with the 8th Division, which had been moving forward from Bukovina since 22 May.

On 23 June, Hungary signed an armistice with Czechoslovakia.

By 4 July, the Hungarian Army had retreated 15 km south of the Hungarian–Czechoslovak demarcation line.

The Allied council demanded that Romania leave Tiszántúl and respect the new borders. Romania said it would only do so after the Hungarian Army demobilized. Kun said he would continue to depend on the might of his army. On 11 July, the Allied council ordered Marshal Ferdinand Foch to prepare a coordinated attack against Hungary using Serb, French and Romanian forces. Hungary, in turn, prepared for action along the Tisza River.

On 20 July, at about 3 a.m., after a fierce bombardment, Hungarian infantry including all three groups crossed the Tisza River and attacked Romanian positions.

On 20 July, in the northern arena, the Hungarians army took Rakamaz and some nearby villages. Troops of the Romanian 16th and 2nd Vânători divisions took back the villages shortly and regained Rakamaz the next day. The Hungarians renewed their efforts and, supported by artillery fire, retook Rakamaz and two nearby villages but could not break out of the Rakamaz bridgehead.

On 20 July, Hungarian forces established a solid bridgehead on the east bank of the Tisza at Szolnok, opposed by the Romanian 91st Regiment of the 18th Infantry Division. The Hungarian army moved the 6th and 7th divisions across the Tisza River, formed up within the bridgehead, then attacked the Romanians in the first line of defense. The Hungarian 6th Infantry Division took Törökszentmiklós; the 7th Division advanced towards Mezőtúr and the 5th Division advanced towards Túrkeve.

On 22 July, Hungarian forces crossed the Tisza River at a point 20 kilometres (12 mi) north of Szolnok and took Kunhegyes from the Romanian 18th Vânători Regiment.

On 21–22 July, Hódmezővásárhely changed hands several times between Hungarian and Romanian troops of the 90th Infantry Regiment supported by the 1st Vânători Brigade. On 23 July, Romanian forces reoccupied Hódmezővásárhely, Szentes and Mindszent.

On 23 July, Hungarian forces took Túrkeve and Mezőtúr.

On 24 July, the Romanian 20th Infantry Division, brought in as reinforcements, cleared the bridgehead at Tiszafüred. Not being able to break out of Rakamaz, Hungarian forces fortified their positions and redeployed some troops. There was a lull in fighting in the north, as the Romanian troops did the same.

On 24 July, the Romanian Army's northern maneuver group attacked. Elements of the 2nd Cavalry Division, supported by troops of the 18th Infantry Division, took Kunhegyes. The Romanian 1st Infantry Division attacked the Hungarian 6th Infantry Division and took Fegyvernek. The Romanian 6th Ddivision was less successful, being counterattacked on the left flank by the Hungarian reserve formations. Altogether, the attack pushed back the Hungarian army 20 kilometres (12 mi). Romanian forces were supported by the 2nd Vânători Division and some cavalry units when they became available.

On 25 July, fighting continued. Hungarian forces counterattacked at Fegyvernek, engaging the Romanian 1st Infantry Division. With their lines breaking, Hungarian troops began a retreat towards the Tisza River bridge at Szolnok.

On 26 July, the Romanians attacked, and by 10 p.m. had cleared the Rakamaz bridgehead. This left the Romanian army in control of the northern part of the Tisza's eastern bank.

After repelling the Hungarian attack, the Romanian army prepared to cross the Tisza River. From 27–29 July, the Romanian Army tested the strength of the Hungarian defense with small attacks. A plan was made to cross the Tisza River near Fegyvernek, where it makes a turn.

On the night of 29–30 July, the Romanian Army crossed the Tisza River. Decoy operations were mounted at other points along the river, bringing intense artillery duels. Romanian forces held the element of surprise.

On 31 July the Hungarian army retreated towards Budapest.

On 2 August, Kun fled Hungary towards the Austrian border and eventually reached the Soviet Union.

Romanian forces continued their advance towards Budapest. On 3 August, under the command of General Rusescu, three squadrons of the 6th Cavalry Regiment of the 4th Brigade entered Budapest.

Until midday on 4 August, 400 Romanian soldiers with two artillery guns held Budapest. Then the bulk of the Romanian troops arrived in the city and a parade was held through the city center in front of the commander, General Moşoiu. Romanian forces continued their advance into Hungary and stopped at Győr. As of 8 August, the Romanians had captured 1,235 Hungarian officers and 40,000 soldiers, seized 350 guns—including two with a caliber of 305 mm—332 machine guns, 52,000 rifles and 87 airplanes.

In early 1920, Romanian troops departed Hungary. They took with them resources including foodstuffs, mineral ores and transportation and factory equipment and also discovered historic bells of Romanian churches in Budapest taken by the Hungarians from Austro-Hungarian Army, which had not been melted then.