in 1916 Romania entered WW1 on the side of the Antant, with the goal to unify all territories, with significant romanian population.

after the 1918 communist revolution in Russia, Romania was left alone on the eastern front - in a desperate situation - so in may, 1918 Romania asked for peace, and dismantled the majority of its army.

meanwhile Hungary suffered from the Aster Revolution, the socialist Károlyi founded the first Hungarian Republic, and completely dismantled the hungarian army.

taking advantage of the worsening situation of the central powers, Romania re-entered the war with the same goal in november 1918, and advanced to Transylvania.

First phase: november 1918 - march 1919

in autumn 1918 after the lost war and the revolutions, order unraveled in Transylvania. the homecoming soldiers and the romanian and hungarian peasants attacked government offices. in the next months the romanian army occupied Transylvania without resistance. the smaller counter movements were retaliated.

in december 1918 the transylvanian romanians proclaimed the unity if Romania and Transylvania. the romanian army moved to the Mures line, what was marked as demarcation line by the Antant. in the meantime the German Army withdrew from Hungary. on demand, the Antant permitted the romanian army to move forward, to the Transylvanian Mountains. the romanian Army moved in to Kolozsvár (Cluj Napocka) and Gyulafehérvár (Alba Iulia). by january 22, the romanian army controlled every territory along the demarcation line.

Hungary still hesitated to sign the peace treaty, so in february the Antant set up a new - wider - demarcation line, starting from the railroads of the Szatmárnémeti (Satu Mare) - Nagyvárad (Oradea) - Arad (Arad) line to Vásárosnamény–Debrecen–Gyoma–Hódmezővásárhely–Szeged, what was the edge of the territorial demands made by Romania.

the hungarian army had to move from the demarcation zone starting march 23 1919. both the inland and military policy of the Károlyi government was a failure. in less than 150 days the hungarian government lost the trust of the people, Károlyi didnt accepted the withdrawal of the army, refused to sign the peace treaty and resigned.

a revolutionary communist government formed in Hungary on march 21 1919 (mainly because the communists had the only fighting groups in the country).

till that point, only minor conflicts happened between the romanian and hungarian army, and one smaller between the ukranian and romanian. some hungarian troops harrassed romanians outside the occupied territories.

Second phase: april 1919 - jun 1919

after march 21, 1919 Romania was bordered by two communist neighbours: the Soviet Union and Hungary. the Antant itself was divided on the communist question: not every member supported military intervention against Hungary. on april 4 they sent another General to discuss the peace treaty. the Antant promised to lighten the terms, if Hungary accepts the Vix-offer.

the new communist leader Kun Béla demanded the romanian army's withdraval to the Maros (Mures) line, so the negotiations stuck. Kun used the time to build a new, highly motivated patriotic army, that can compete with the czechslovak and romanian army. he also hoped the Soviet Union will come to aid Hungary.

because of the stucked negotiations, the romanian armies (north and south) planned to capture Nagykároly and Nagyvárad (Oradea), to cut the Székely troops from the hungarian army. they planned to start the offensive to april 16, 1919.

the hungarians found out about the preparations, strenghtened the defense lines, and started a preventive strike on april 15. the romanians - using reserves - repelled the hungarian attacks, and moved in defensive positions.

between april 16 and 18 the romanians started their own offensive. the first phase of the offensive ended, when the romanian troops broke through the hungarian front on april 18. on april 19 romanian troops occupied Nagykároly and on april 20 Nagyvárad (Oradea) and Nagyszalonta. they reached the demarcation line, but for military purposes the romanian army moved forward to the Tisza line. meanwhile bloody attrocities against hungarian civilians were common.

the romanian cavalry hindered every hungarian effort to build another front line. the best fighters of the hungarian army, the Székely Corps tried to stop the advancing romanian army at Nyíregyháza, but the city was lost and occupied on april 24, 1919, and the encircled Székely Corps surrendered on april 29.

Debrecen was occupied on april 24, and the romanian army started to plan the siege of Békéscsaba. the siege started on april 25, and after a short fight the city was occupied on april 26. the remnants of the hungarian army fell back to Szolnok, tried to cross the Tisza river, and built two defense lines around the city.

between april 29 and may 1 the the romanian army - despite the hungarian reinforcements - broke through these defense lines. by may 1, 1919 the whole east strand of the Tisza river was controlled by romanian forces.

on may 2, 1919 the hungarian government asked for peace, promised to recognize romanian territorial demands, in exchange for cease fire and no intervention in hungarian inner politics. romanians only offered cease fire, and only on pressure from the Antant.

at the Paris peace negotiations, on april 30 the romanian PM promised to the Antant, not to cross the Tisza line. General Moșoiu was appointed as the governor of the Tisza - Romanian Border territory. some romanian soldiers were sent to North-Moldavia from the occupied territories.

using the silence. Kun prepared the army against Czechslovakia, to imporve Hungary's position in the peace treaty. it was a logic decision. Serbia was no option, because of the big french military presence, and Romania just beat Hungary.

on may 20, 1919 hungarian troops attacked the czechslovak army, stationed at Miskolc with full force. the romanian army tried to stay in contact with the czechslovak army, and attacked the hungarians from the side, but their operation was a failure, the czechslovak army had to flee from Hungary.

romanians moved back to their HQ in Tokaj, and successfully defended it between may 25 and 30. the expanding hungarian attacks against the czechslovak army made the romanian army fall back behind the Tisza line, destroying every bridge on the way. the romanian army extended its defensive positions, and connected the south and north frontlines to hinder communication between the Soviet Union and Hungary, and encirclement.

the military success made Hungary possible to regain some of its former territories, and set up a satellite state in Slovakia, called the Slovak Soviet Republic. Hungary started to prepare for an eastern offensive against Romania.

april 9, 1919 Bessarabia united with Romania, what Russia did not recognized. it was a source of conflict between Romania and Russia, but Russia suffered from civil war, and was fighting against Poland and later Ukraine, so was no match for the romanian troops, stationed in Bessarabia.

to ease the pressure on the hungarian government, the foreign minister of the Russian bolshevik government sent an ultimatum to Romania, so some romanian troops were sent to Bessarabia from Hungary.

edit: typo

Reddit didnt allowed the longpost:

Third Phase: juli 1919 - august 1919

the Antant disliked that Romania moved to the Tisza line. members accused Romania for the chaotic hungarian situation, and demanded the romanians to fall back to the demarcation line. the romanian government held out, and didnt pulled back.

the Antant threatened Hungary with a joint attack from Serbian-Romanian-Czechslovakian-French forces, if Hungary continues its northern campaign. also promised a more favorable peace condition, which was presented to the parties on juni 12, 1919.

with these conditions, Hungary signed cease fire with Czechslovakia, and moved its troops behind the demarcation lines. the romanian army was also ordered to fall back to the demarcation line, but the romanian government asked the dismantling of the hungarian army in exchange.

that was no option, so Kun stated he will only rely on the strength of its army from now on. that turned the Antant against Kun, and they decided on juli 11, 1919 to start their joint attack against Hungary. after the cease fire treaty with Czechslovakia, Hungary deployed its troops to the eastern front, and started with a preventive strike on juli 17, 1919.

the romanian army faced the hungarian army along a 250 km long frontline. south they were connected with the Serbian-French armies, north they had connection with the Czechslovak armies. the hungarian army was highly motivated, was set up in three divisions, (north, middle, south) and planned to cross the Tisza river, what - with an attack from the Soviet union on Bessarabia - will push Romania in a revolution.

the romanian army was also highly motivated, and was set up in 3 defensive lines. the first line was the weakest, its purpose was to explore the real tactic of the hungarian army. the second line was somewhat stronger to hold up the hungarian army, and the strongest 3rd lines duty was to counter the hungarian army.

hungarians were bombarding the romanian positions between july 17 and 20, and on july 20 all three hungarian divisions crossed the Tisza. hungarians on the northern front captured Rakamaz, but were repelled on july 26, and so the romanian army controlled the northern tisza line again.

the southern front captured Szentes in two days, but the romanians captured it back the next day, repelled the hungarians, what ended the southern front, so romanians redirected the troops to the "middle". hungarians were gaining ground on the middle front, on july 22 hungarians moved forward to Kunhegyes, and occupied Mezőtúr and Túrkeve on july 23.

romanians started the counter offensive on july 24, occupied Fegyvernek, and pushed hungarian troops 20 km back. fights continued around Fegyvernek on july 25, hungarians started a counter-counter offensive, but the romanian army broke through the hungarian lines on both north and south fronts. the hungarian army started to retreat, the whole Tisza line was under romanian control again by the evening of july 26.

after they repelled the hungarians, the romanian government decided to cross the Tisza line and give the coup-de-grace to the Hungarian Soviet Republic. hungarians bombarded the receptables of the romanian army, but the night between july 29 and 30, the romanian army crossed the Tisza at Fegyvernek, so the hungarian army gave up the Tisza line and moved towards Budapest.

the romanian army started to push towards Budapest. the biggest fights happened on the southern front on august 1, around Szolnok. hungarians on the southern front surrendered on august 1. the encirlced hungarian forces in the middle and northern front surrendered on august 3. by the evening of august 3, 1919 the Hungarian Red Army was no more.

Kun left Hungary on august 2. Budapest was occupied with 400 men and two cannons for a good half day, when the rest of the romanian forces arrived and entered in a triumph march.

on august 11, 1919 Archduke Joseph received a very short termed ultimatum, and was asked to give all the military equipment to the romanians, and form a personal union with the Romanian King, who will be the new ruler of Hungary. Archduke Jospeh had to resign from the hungarian throne, and save Hungary from a personal union.

romanians continued to push forward till Győr, they practically occupied whole Hungary till the beginning of 1920, except the Balaton region, where Horthy - with romanian weapons and help - prepared his national army for his rise in power.

edit: typo2