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.