Hungary was in war even before Germany again after WW1. Hungary (under the communist rule of Bela Kun) invaded the CSR and occupied Slovakia/Upper Hungary until 1919
https://en.wikipedia.org/wiki/Hungarian–Czechoslovak_War
hoping for soviet aid that never materialized. Two additional month the hungarian-romanian war took
https://en.wikipedia.org/wiki/Hungarian–Romanian_War
and romainan troops occupied eastern Hungary until 1920.
So Hungary was very interested in revising the new borders after WW1 and just waiting for an opportunity to do so without any major power intervening. Which is exactly the situation after Locarno and Munich where the western allies have practically left the CSR and Poland on their own for a while and Germany and Italy mediate the hungarian demands on the CSR.
And even in the Vienna Award negotiations Hungary wanted at first to go beyond the german suggestion for a new border
https://en.wikipedia.org/wiki/First_Vienna_Award
The Bela Kuhn regime was financially backed, and in some cases trained, by the Soviets, seizing power by assassination and intimidation. The former government had mostly resigned after signing the Trianon Treaty under protest, feeling that they had shamed themselves and their country by accepting the unacceptable, so there was very little actual "government" when the revolt broke out. Hungary was not even allowed to mobilize in order to put down the revolt. That government was eventually removed by a combination of French-supported Romanian troops, as well as unofficial Hungarian forces consisting primarily of WWI veterans, and most of the former leaders of the regime were either killed during the fighting or imprisoned afterwards. Until that point, the return of the lost territory and people by any means was considered of paramount importance within the country, and Bela Kuhn and his cabinet had tried to gain popular support by provoking a conflict.
The following regime, under former A-H Admiral Miklos Horthy, followed a more peaceful policy of petitioning in the League of Nations (under Article 14) for plebiscites in the former Hungarian territories. The French and Czechs in particular were highly antagonistic toward any changes (peaceful border adjustments as detailed under Article 14, as well as any forceful changes under Article 9), as it might set a precedent for their other integral minority groups and/or overseas colonies, and all of the proposals were denied and buried without a formal hearing. Benes in Czechoslovakia openly threatened to invade disarmed Hungary at one point for its "warmongering", and France agreed to "stand by its commitments" with Czechoslovakia in the event of war. Funny how France didn't offer to stand by those same commitments at Munich. After Munich, any cooperation between Czechoslovakia and Poland was practically impossible, as Poland had become a German accomplice by demanding and receiving some disputed Czech land under the treaty.
After Munich, when Hitler and Mussolini offered to mediate the dispute between Hungary and Czechoslovakia, it was a difficult decision for Horthy and the Hungarian parliament: do you make a deal with the devil to get back your land and people, or refuse and consign them to perpetual second-class status in another country, whether it be Czechoslovakia, Slovakia, or Germany? The additional threat factor, balancing fear of Germany against an even bigger fear of the Soviets, was another thing to consider, and prominent in the minds of the Hungarian leaders. Most of this is covered by former US Ambassador to Hungary J. F. Montgomery's book "Hungary, the Unwilling Satellite", detailing his term as Ambassador until Hungary's entry to the war, and his amicable but necessary departure from the country. The Appendix section includes notes from several letters from Swiss diplomats still in the country shortly after Hungary's surrender, and the looting of Budapest by the occupying Soviets.
In the end, Hitler turned over Czechoslovakian land to Hungary: not all of the former Hungarian land that was asked for, but with a LOT of additional unrequested and non-Hungarian territory in Ruthenia, which Hitler wanted in neutral hands in order to prevent Poland from receiving support from Romania, if/when Hitler decided to pursue that course. The First Vienna Award was calculated by and for the benefit of Germany, with enough benefit for Hungary to all but force its acceptance, but with no further discussion of the terms allowed for either the Hungarian or Czech ambassadors..."shut up and sign it". Hungary's complicity in the destruction of Czechoslovakia insured their acceptance or even grudging support for further German expansions.
Ultimately, Poland, Czechoslovakia, Hungary, and most of the other Eastern European and Balkan countries acted for short-term and selfish reasons, and while they were clearly concerned for the longer-term consequences of their actions, they all took advantage of the "shiny" opportunities as they were presented. Hitler played on their rivalries and animosities, setting them up to fall one by one like dominos.
Looking back at the 100th Anniversary of the end of WWI, that flawed document set up the disastrous course of events of the mid-20th Century. It did not reestablish a fair balance in Europe, yet was not harsh enough to prevent Germany from wanting its land and "Great Power" status back. Wilson was correct that this was not a fair solution to the problem, and that it would not lead to lasting peace. We can see all of the conflicts which resulted in Germany, Yugoslavia, Czechoslovakia, and the Middle East over the next hundred years as a direct consequence of that short-sighted document.
