The main cause of this war can be attributed to the fall of the democratic Hungarian government to the socialists under Bela Kun on February 7, 1919. In the weeks that followed, Bela Kun established his brand of socialism, dividing up the land among the peasants, among other things. The Russian government started to get worried. A socialist nation right on the border with Russia could provoke the peasants in Russia to revolt against the Monarchy. They began preparing to take preventive measures against the Kun government in Hungary. Divisions that were going to be released back to civilian life were called to the Hungarian border, and supplies were stockpiled along the roads toward Hungary.
The Hungarians saw this and began to set up defensive lines on the Russia border. There was mass conscription, and the Kun government began stockpiling their own supplies for a long struggle. They also contacted some of their people in Russia and authorised them to begin plans for staging peasant uprisings against the Monarchy. These men would mostly be stuck by themselves, as the Kun government would be otherwise occupied by the war with Russia.
The Opposing Strategies
The Russian forces were placed under the leadership of Aleksei Brusilov, a promising soldier who had proved his brilliance in the later stages of the Third Balkan War. This would be his first time commanding a full scale offensive, and Russia was betting that he would not fail.
The Russians planned to try and defeat Hungary with minimal Hungarian civilian casualties, in order to advance their plans for a Balkan power bloc. Although, any civilians known to be socialist collaborators were to be quietly removed from their residences and deported to Russian work camps.
Brusilov decided that the Hungarians would probably fight a defensive war, as they didn't have the population to do large scale offensives. With that in mind, he decided to implement a new tactic that he called Infiltration. This could best be summarized in a few points: Getting the trenches close, attacking over a wide front at multiple areas, and short artillery bombardments aimed at cutting the barbed wire between trenches. This was developed near the end of the Third Balkan War, when the Austrian defenses were starting to fall apart, and never used in a large scale offensive, so it was not known how effective it would be against a prepared enemy. With that in mind, Brusilov decided to take the plunge.
The Hungarians knew that there was no chance of a large scale offensive war against Russia. Taking the lessons from the Third Balkan War. They decided that their best bet would be to stage defense in depth, with multiple defensive lines right after another. They bet on the fact that if they could make Russia pay massive casualties to gain a little bit of ground, then eventually war-weariness and peasant uprisings would bring down the entire Monarchy. The constraints to this plan was a shortage of ammunition and guns, but they figured that they could hold out long enough to overcome that.
The Beginning of War
Russia sent an ultimatum to Hungary on March 1, 1919. The ultimatum demanded the dissolution of the socialist Hungarian government, and that Bela Kun be turned over to Russian authorities for trial on terrorist related charges. Naturally, these demands were rebuffed. With the gauntlet down, on March 15, Russian forces crossed the border into Hungary. They managed nine mi through the Hungarian side of the Carpathian mountains before the Hungarian defenses started scything Russian troops down in droves. At this point, Brusilov called off the offensive in order to set up opposing trench lines, in preparation for the initiation of his infiltration doctrine. However, events would interfere to prevent the quick downfall of the Hungarians.
The Great Peasant Uprising
The Hungarians had armed radicalists within Russia, and told them to rise up when the time was right. That time came when a peasant man was beaten by Russian soldiers for an extremely small incident. This expanded into a general call for revolution and overthrow of the monarchy by the Russian socialists and other armed radicalists. the Russians had to withdraw forces from the trench lines to crush the revolution, forcing Brusilov to hold off his offensive. The Russian troops arrived in St Petersburg, where fighting raged in the streets between police and rioters. The arrival of soldiers tipped the scales in favor of the monarchy, and the revolters were defeated. With the monarchist victory in St Petersburg, the other cities experiencing revolts are able to subdue their rebellions.
While the battles raged across Russia between rebels and soldiers, the Hungarians launched an offensive, going against their chosen strategy, in an attempt to drive the Russians off of Hungarian soil. Hopefully a victory here would encourage more people to rebel in Russia, leading to victory. The Hungarians launched their attack in the manner that they had in the previous war, using precious artillery shells to blast holes in the enemy fortifications. Brusilov quickly realized what the Hungarians had in mind, and decided not to oblige them. He pulled out most of his troops from that area, and posted them on the flanks. He also ordered his artillery to not try and blast down the Hungarian's artillery, instead saving it for the incoming assault.
The Hungarians attack in the area that they had been shelling, making good progress for the first 5 feet. At that point, Brusilov orders up the artillery that had been silent. The Russian guns work a slaughter among advancing Hungarian troops, who had figured that they would be unable to respond. Hungarian artillery opens up again in an attempt to stop the Russian artillery. the Russians turn to blasting apart the Hungarian artillery, letting the offensive continue. The soldiers pierce through the first trench line, but then the Russians on their flank open up with machine guns, giving them hard losses. Despite the moral of their soldiers, the Hungarian offensive was repelled within a week due to the sheer firepower of the Russians. It was now Russia's turn to hit the Hungarians.
The Russian Offensive
With the return of the Russian troops who had subdued the revolts in St Petersburg and around Russia, Brusilov was ready to launch his offensive against the Hungarian trench lines. He kept them a few miles away from the trenches themselves, in order to keep the Hungarians from realizing what was coming. The Hungarians are fooled, as they do not have airplanes, and any observation balloons are quickly shot down, thus preventing them from observing the Russian lines in depth.
Brusilov launched his offensive on April 9, with a short twp hour artillery bombardment mostly aimed at removing the barbed wire, and weakening fortifications. With the shelling over, the Russians jumped off, crashing into the Hungarian lines. They were able to pierce through, and start to make strategic depth. Brusilov released his reserves to the breakthrough to keep the pressure up on the Hungarian troops. The Russians started rolling up the line on their flank, and the Hungarians started falling back toward Budapest. They tried to establish more defensive lines, but Brusilov kept up the momentum, and they were unable.
Finally, the Hungarians drew Brusilov into a devastating city battle inside Budapest. The Hungarian government had already evacuated to Veszprem, and were in no danger of capture. After a short while, Brusilov forced a crossing of the Danube, and encircled Budapest, cutting it off from supplies. The Hungarians in the city fought on, but lack of ammunition and food eventually caused them to surrender.
While the Hungarians in Budapest suffered under continuous Russian attacks, Brusilov had also moved his forces toward Veszprem. The Hungarian army was hard pressed to stop them, and when Budapest fell, even the most blind could see that it was over. The Hungarians had nothing left with which to fight off the Russians, who would be receiving re-inforcements from the soldiers formerly besieging Budapest. A coup attempt against the Socialist government failed, but Bela Kun realized that he could not stay, and fled through Austria into Switzerland.
The End of War
The new, pro-Russia, government immediately began negotiating for an end to the war. The Russians were angry that they had let Kun escape to Switzerland, but realized that there was nothing they could do about it right now. They demanded that Hungary sign an alliance with Russia, and pay a small sum for the money Russia had to spend on this war. The Hungarians agreed, as they had no choice.
The outcome of this war helped to polarize the Balkans into either the German or Russian camps. If a war started between the two powers, this area would immediately go up in flames. It also helped the rising star of Brusilov, who was made Commander of the Baltic District. The Russians now had a general who could defeat the tactics of the trench line, and in a quick matter. The stage was slowly being set for World War 1.