Ok, here's the story, and my ideas! I translated what I found on Hungarian wiki, maybe not the best translations, but I hope you can understand it
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The Hungarian-Yugoslav eternal friendship agreement signed in 12th december 1940 at Belgrade. (It was not ratificating so it was never come into force).
Premise
Before the agreement Teleki’s government prepared for a war which might been brake out between Romania and Hungary which lost Transylvania after World War 1, and willing to regain it. Teleki’s government wanted to stay netural and don’t pledge themself to the more and more powerfull Germany, so they started to search other diplomatic contacts. Yugoslavia had a very good relations with United Kingdom despite they were also had good relations with Germany.
For Yugoslavia it was important too, that with a Hungarian help they can moderate German influence and the international isolation in the Balkans which one was caused because Yugoslavia’s friends, Czechoslovakia and France became under German rule, Romania became German friend while Yugoslavia’s neighbours, Bulgaria and Italy became belligerent.
The approach’s priority was so close, that in the end of 1939 in the Hungarian press and forums often occur the wish to settle the Hungarian and Yugoslavian relations friendly. This conception was supported by Endre Bajcsy-Zsilinszky with others too.
Bajcsy-Zsilinszky’s friend, Imre Németh was arrived to the Balakan-conference in 9th december into Belgrade and informed the Yugoslav prime minister Dragiša Cvetkovićot and other Yugoslav politicans about this morale in Hungary. In the return to Budapest, Németh informed Teleki about the favourable welcome. The „Magyarország” titled gazette published a report about this approach with „Hungarian word, Hungarian song, Hungarian morale in Belgrade” title. Following this Teleki cheered Bajcsy-Zsilinszky to go to Belgrade and continue the exploratory talk. The approach supported by Vladko Maček too. During the discussions Bajcsy-Zsilinszky on the side of he’s many arguments he refered Lajos Kossuth’s conception about the „Dunavölgye” confederation.
This agreement supported by Germany too, because Hitler trusted in Yugoslavia’s german friendship and while he started to plan a war against USSR he wanted to know Balkans in safe.
Signing the agreement
The agreement which is signed in 12th december 1940 contained only 3 points. The first 2 points:
1. Between the Hungarian and Yugoslav kingdom there will be always an eternal friendship and peace exist.
2. The signatories to this agreement agrees about that they will always consult about the questions which is concern they relations.
In the agreement Hungary didn’t put away the territorial claims on Yugoslavia, to regain Vojvodina/Vajdaság. About this point there was a bit conflict between Germany and Hungary. Because Germany also want to sign an agreement with Yugoslavia and it would contain that Germany guarantee the territorial integration of the Yugoslav kingdom. The Hungarian foreign minister, László Bárdossy wag one’s finger at German foreign minister Ribbentrop during it’s visit in Berlin so Ribbentrop promised that they will only „respect” the territorial integration of the Yugoslav kingdom.
The first point of this agreement raised moraly very high the two country’s relation which was uncommon in international agreements. Lately in april 1941 this agreement critisisd a lot and was very emberassing for Hungary.
Breaking the agreement
The chaos around Yugoslavia’s internal affairs and the German power demands was put the approach of the two country’s in to opposite way. In 26th march 1941 Yugoslavia joined the 3 power agreement to Germany, Italy and Japan, but the next day an anti german coup removed Cvetković’s government.
Germany felt it is now a good time to invade Yugoslavia, and they wanted Hungary as a station during the invasion. For this Germany exploited Hungary’s territorial claims on Yugoslavia. Invading Yugoslavia was turned away the creation of little-antant but it was meant breach of promise.
In this hot water Teleki first resigned, but then recalled it’s resignation and tryed to stop make war on Yugoslavia. The Supreme War Council accepted the proposal in 1th april 1941 to only go to war if Yugoslavia end as a state. In the next day Miklós Horthy and the main chief of the army told Teleki that they will not recognise the decision fully.
The broody prime minister’s situation was became even worser, when the United Kingdom told him that they will not excuse any agression against the Yugoslav kingdom.
In 3th april at around 1:30 am and 2:30 am in the Sándor palace Teleki in he’s room shot himself on head. „We were became truthless. We joined the side of villains. I wasn’t hold you back, I’m guilty.” – write Teleki on he’s suicide note to Horthy.
Teleki’s action was impressed Winston Churchill very deeply so he said: When the war’s over on the treaty conferences we must leave a seat empty to Teleki.
The full text of Teleki’s suicide note to Miklós Horthy:
„My lord!
We were became truthless – because of cowardice – opposite the eternal friendship which was conjectural of the Mohács speach. The nation feels this, and we threw our honor.
We joined the side of villains – Because none of the fictitouis atrocitys were true. Nor against Hungarians and nor against Germans! We will became grave-robbers! The most grottiest nation!
I wasn’t hold you back!
I’m guilty!
Pál Teleki
3th april 1941”
There was an other version, if Teleki won’t commit suicide:
„My Lord!
If my action was not successfull, and if I live, I resign ASAP.
in all submissiveness
Pál Teleki
3th april 1941”
Ok, so that's for the story, now here's my ideas:
If the player plays as Yugoslavia, everything happens accord to history, or it maybe a chance to change the situation, if for example the relations between Hungary and Yugoslavia is +200. Then it would be something like, that Hungary won't attack Yugoslavia but don't go to arms on the side of Yugoslavia either, only sends aid and accept refugees (just like when Poland invaded by Germany, Slovakia, USSR, Hungary wasn't attacked Poland, because Teleki said "it's a question of national honor to not attack Poland from the territory of Hungary." /Hungary and Poland have a very very big friendship which is often called brothership, despite we, Hungarians are not slavs / Hungary accepted Polish refugees, and gave every help to them, but was not attacked Germany, it would be a suicide.)
As playing Hungary, there would be events, to sign the agreement or not sign the agreement, and depends on this, relations can improve between nations.
Sign the agreement:
relations with Yugoslavia: +100
with United Kingdom: +75
with Germany: +50
Don't sign the agreement:
No effect
After signing the agreement, the begin of the invasion of Yugoslavia, there would be options:
Keep our agreement with Yugoslavia and declare war on Germany:
relations with Yugoslavia: +150
with United Kingdom: +100
with Germany: -75
Keep our agreement with Yugoslavia, but stay netural and don't start war:
relations with Yugoslavia: +50
with United Kingdom: no change
with Germany: -50
Keep our agreement with Yugoslavia, and send aid, weapons to Yugoslavia and accept refugees:
relations with Yugoslavia: +100
with UK: +50
with Germany: -25
A mount of resources send to Yugoslavia, and a groth of manpower because of refugees go to Hungary.
Brake the agreement, and declare war on Yugoslavia, accept the will of Germany:
Relations with Yugoslavia: -200
with UK: -200
with Germany: +100
count Pál Teleki no more available
That's all for now, if other Hungarians would have other ideas I think it would be cool because I'm not very sure if I was accurate enough.
Daemoncool said:
As playing Hungary, there would be events, to sign the agreement or not sign the agreement, and depends on this, relations can improve between nations.
Sign the agreement:
relations with Yugoslavia: +100
with United Kingdom: +75
with Germany: +50
The relation increase with UK might be much lower. Like +50 or so. As for germany a +20 would be enough.
Daemoncool said:
After signing the agreement, the begin of the invasion of Yugoslavia, there would be options:
Keep our agreement with Yugoslavia and declare war on Germany:
relations with Yugoslavia: +150
with United Kingdom: +100
with Germany: -75
This suicide. So it should have 0 AI chance. Please note that the agreement was not to help each other militarily, but not to attack each other.
Daemoncool said:
Keep our agreement with Yugoslavia, but stay neutral and don't start war:
relations with Yugoslavia: +50
with United Kingdom: no change
with Germany: -50
Also leave Axis is HUN is already in. This would be a major breaking point.
Daemoncool said:
Keep our agreement with Yugoslavia, and send aid, weapons to Yugoslavia and accept refugees:
relations with Yugoslavia: +100
with UK: +50
with Germany: -25
A mount of resources send to Yugoslavia, and a groth of manpower because of refugees go to Hungary.
Giving help might be too much. And the refugees as manpower should go to the UK (like the Polish ones) or to other big Allied Nation. Or should give some bonuses to partisan activity afterwards.
Daemoncool said:
Brake the agreement, and declare war on Yugoslavia, accept the will of Germany:
Relations with Yugoslavia: -200
with UK: -200
with Germany: +100
count Pál Teleki no more available
HUN joins the Axis.
And since Armagedon suports additional TAGS, I don't see a problem. 
