After the breakup of Austria-Hungary at the end of WW1, tensions between Romania and Hungary were at an all-time high, culminating with the Hungarian-Romanian war of 1919 and the defeat of the Hungarian Soviet Republic and the occupation of Budapest by the Romanian Army. During this time, Romanian troops occupied Carpathian Ruthenia on behalf of Czechoslovakia until August 1919.
In October 1921, as it seemed that former Habsburg Emperor Karl might succeed in regaining his Hungarian throne, Romanian ministers declared that such a move would be regarded as a cassus belli.
Therefore, a conflict between Romania and Hungary in the 30s is not entirely ASB.
In March 1939, with the breakup of Czechoslovakia, Carpathian Ruthenia declared its independence as the Republic of Carpatho-Ukraine. The same day, Hungary invaded. Some fighting between Czech units and Ruthenian irregulars against the Hungarians took place, but these were massively outnumbered and forced to flee to Romania.

So, what if on March 15 or as late as the early hours of the 16th, the Ruthenians ask Romania for help ? Naturally, a different Romanian leadership would be required in order for this to work, but it's nothing a well placed POD can't solve, such as a different personality of King Carol II or a failure of the national peasants party to win elections, leading to a continuation of King Michael's rule, with real power in the hands of liberal prime minister Bratianu, or even something else entirely.
Regardless of the exact POD, assume we have a situation where the Romanian leadership decides that Hungarian expansion has to be stopped, that Germany would not risk war over it and, even if it did, it would be quickly defeated by the French, leading to a better future for Europe. In the early hours of March 16th, the Romanian King crosses the border into Carphato-Ukraine and is elected king in the small town of Tiacziw just across the border (thus creating a personal union of the 2 countries). At the same time, Romanian army units start pouring into Ruthenia and engage advancing Hungarian columns before these have a chance to occupy the capital of Chust.
While the fighting would be pretty small scale and confused, the very poor quality of the Hungarian army (compared to the only poor quality of the Romanian army), plus the fact that the Hungarians only had ammo for about a day of sustained fighting, would eventually lead to a situation where the Hungarians are repulsed.
So, assuming this or something similar happened, what would Hitler do ? Would he go to war over the issue ? If so, what would the Allies and Poland do ? And since this is only March 1939, can we expect a German defeat or would they still occupy and/or dominate mainland Europe ?
In October 1921, as it seemed that former Habsburg Emperor Karl might succeed in regaining his Hungarian throne, Romanian ministers declared that such a move would be regarded as a cassus belli.
Therefore, a conflict between Romania and Hungary in the 30s is not entirely ASB.
In March 1939, with the breakup of Czechoslovakia, Carpathian Ruthenia declared its independence as the Republic of Carpatho-Ukraine. The same day, Hungary invaded. Some fighting between Czech units and Ruthenian irregulars against the Hungarians took place, but these were massively outnumbered and forced to flee to Romania.

So, what if on March 15 or as late as the early hours of the 16th, the Ruthenians ask Romania for help ? Naturally, a different Romanian leadership would be required in order for this to work, but it's nothing a well placed POD can't solve, such as a different personality of King Carol II or a failure of the national peasants party to win elections, leading to a continuation of King Michael's rule, with real power in the hands of liberal prime minister Bratianu, or even something else entirely.
Regardless of the exact POD, assume we have a situation where the Romanian leadership decides that Hungarian expansion has to be stopped, that Germany would not risk war over it and, even if it did, it would be quickly defeated by the French, leading to a better future for Europe. In the early hours of March 16th, the Romanian King crosses the border into Carphato-Ukraine and is elected king in the small town of Tiacziw just across the border (thus creating a personal union of the 2 countries). At the same time, Romanian army units start pouring into Ruthenia and engage advancing Hungarian columns before these have a chance to occupy the capital of Chust.
While the fighting would be pretty small scale and confused, the very poor quality of the Hungarian army (compared to the only poor quality of the Romanian army), plus the fact that the Hungarians only had ammo for about a day of sustained fighting, would eventually lead to a situation where the Hungarians are repulsed.
So, assuming this or something similar happened, what would Hitler do ? Would he go to war over the issue ? If so, what would the Allies and Poland do ? And since this is only March 1939, can we expect a German defeat or would they still occupy and/or dominate mainland Europe ?