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.
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 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 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.
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.
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.
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.