No country was perished more viciously in Trianon than Hungary. But this country is dwelled by spiritually strong people, who won’t be resigned to the demolition of their country. Hungary’s dismembering is so dishonourable that no one takes responsibility for it. Everybody acts like they don’t know about it, everybody is in coy silence. The reference to the right of nations’ self-determination is only an untrue formula… they misused their victory in the most vicious way… There’s no French, English or Italian who would accept the conditions forced to Hungary for their own nation…
Until the end of the Austro-Hungarian in 1918 and the Treaty of Trianon in 1920, the official borders of Hungary roughly mirrored those of the Medieval Kingdom of Hungary. The history of Hungary after the death of Matthias Corvinus, who arguably brought the Kingdom to its apex, is a story of fall, resurrection, and ultimately of dismemberment.
After the death of Corvinus, Hungarian nobles elected a foreign King, Lajos II, who was no more than a boy, and dissolved the King's hated tax system and the royal army that it funded. In 1526, acting upon the perceived weakness of Hungary, the Ottomans invaded the country and defeated its entire army at Mohacs. Sultain Suleiman Kanuni of the Ottomans reportedly waited for a day at the battle site, expecting a second attack. He could not believe reports that the army he had crushed - less than a third of the size of his own, with no modern equipment and fighting with outdated equipment and tactics - was the full strength of the once mighty Kingdom of Hungary.
Of course, while the central government of the Kingdom of Hungary had been dismembered by this victory, the outlying provinces were not. Hungarian nobles continued to rebel against Ottoman rule, and momentarily established states, the largest of which was John Zápolya's state in Transylvania. In November of 1526, Holy Roman Emperor and Archduke of Austrian Ferdinand II was declared King by much of the remaining Hungarian nobility, contesting both the Ottomans and Zapolya. Ferdinand defeated Zapolya in two battles, forcing Zapolya to take refuge with Suleiman, and vassalizing Hungary to the Ottomans. War would rage over Hungary between Austria and the Ottomans until 1683. In that year, a resurgent Ottoman Empire led by Vizier Kara Mustafa Pasha raised a massive army to besiege Vienna. However, they were defeated in a series of battles by the brilliant Prince Eugene of Savoy. The resultant peace, the Treaty of Zenta, turned over most of medieval Hungary to Austria.
However, while Hungary became subordinate to Austria, it was claimed as a hereditary possession, not as an imperial province under central control. In other words, Austrian claims to Hungary derived from the election of the Austrian Archduke as King of Hungary, and, in an age where legitimacy was a vital point of royal diplomacy, Hungary was never partitioned into smaller provinces, despite around 60% of those living in the "Kingdom of Hungary" being non-Hungarian speakers.
Hungary lived as a subordinate but important appendage of the Kingdom of Austria until 1866. Up to that point, its manpower had been vital in rescuing the Habsburgs from the crisis of the War of Austrian Succession, and from the Napoleonic Wars. Far from being oppressed, the Hungarian magnates - the country's largest landowners - were always key in Habsburg governments, and owned some of the largest estates of Europe, part of Europe's highest social class. However, as nationalism spread through Europe after the Napoleonic wars, lesser Hungarian nobles and the middle class grew increasingly discontent with their nation's subordinate status, and that many of its laws were legislated from Vienna. In 1848, the Hungarians attempted to revolt, but were crushed by a coalition of Austrian, Russian, and Croatian forces. In 1866, however, Austria suffered a horrible defeat at the hands of Prussia, and relented to give the Hungarians autonomy in their own lands and almost equal status to Austria.
From 1866 to 1918, Austria and Hungary were united in a common economy, in which internal trade and regional specialization were important. Czechia was the most industrialized part of the Empire, while the Hungarian plain was the Empire's breadbasket, producing 500% more food than it required per annum. These economic relations, and the common interest of Austrian and Hungarian elites in repressing independence movements from the other minorities in the Empire - Czechs, Slovaks, Italians, Serbs, Croats, Slovenes, and Romanians - kept the Empire together. Despite old tensions, Austria and Hungary remained together because the union made sense. There were, however, still points of antagonism and inequality. Austria and Hungary, aside from the central royal army, had their own separate forces, known as the Landwehr and the Honved. The Honved was notoriously forbidden to have artillery, in fears that it might separate.
For the most part, Hungary pursued its own domestic policy and was a relatively content partner in this relationship, coming the closest that it had since Mohacs to restoring some semblance of independence.
In 1918, all that ended. Hungary declared the Austro-Hungarian union dissolved after a reformed Italian army crushed the royal forces in the last battle of the Isonzo, aided by a giant artillery park. In a series of armistice agreements, Hungary agreed to surrender territory. Vojvodina and much of Croatia and Slovenia were surrendered to the Serbs. The Czech legions occupied most of Slovakia, and Romania retained a part of Transylvania at the end of the war. Allied powers had begun to negotiate a partition of Hungarian lands without Hungarian participation.
In 1919, the allies demanded that Hungary accept a new boundary with Romania at the river Tisza. Unable to refuse but unwilling to accept, the Hungarian provisional government resigned, and the Communists took power. The Communist Red Army launched a successful invasion of Czechoslovakia, taking territories and signing a ceasefire. Their invasion of Romania, however, was less successful, and their forces collapsed after a week of fierce fighting. The Romanians took Budapest and installed Regent Miklos Horthy.
In 1920, the terms of the Treaty of Trianon spelled out. Hungary lost three quarters of its prewar territory, and one third of ethnic Hungarians were outside Hungary's borders. Worse yet, the interdependent economy of Austria-Hungary was shattered by the creation of new borders. In each of the Austro-Hungarian successor states, rural unemployment ranged between 25 and 33%.
The Hungarian delegation to the peace conference made these points clear, and Woodrow Wilson of the United States was sympathetic. However, the British and French would have none of it. They had three priorities. The French saw Romania as a close historical and cultural partner, and looked out for their interests. Second, Romania and Serbia had suffered massively in the war - the manyfold territorial expansions to both states after the war, even at the expense of Italy in the case of Serbia - were rewards for the immense personal sacrifice that both nations endured in fighting the Central Powers on abandoned, hopeless fronts until the bitter end. Finally, the Anglo-French interest was in preventing the resurgence of Germany. They feared that a renewed Austro-Hungarian union could become a German proxy that would re-assert itself in the Balkans, and thus they sought to strengthen Czechoslovakia, Romania, and Yugoslavia such that either state would be stronger than Austria or Hungary.
Basically, Hungary ceased to exist as a completely independent power in 1526, when much of the nobility threw its lot in with Austria. It regained a measure of self-rule in 1866. Its archaic Medieval borders, which did not reflect ethnic realities, were preserved because of the legitimacy concerns of the Habsburg monarchy which owned the crown. Come the end of the First World War, "national self determination" as one of Wilson's 14 points meant non-Hungarian lands would have left regardless, but the need to reward Romania and Serbia, which fought a long and hard war, and the desire to contain former German allies, meant the allies awarded neighboring states with even larger stretches of land.
Sources:
Teszelesky, Robert. The Matthias Corvinus cultus in early modern Hungary
Judson, Pieter. The Habsburg Empire.
Merkin, Yakov. Crosscurrents: Navigating The Turbulent Politics Of The Right During The Horthy Era In Hungary, 1920-1944.