NATO

The North Atlantic Treaty Organization (NATO, French: Organisation du traité de l'Atlantique nord, OTAN) is a military alliance consisting of 27 European, 2 North American, and 1 Eurasian country formed in 1949, after WW2 has ended, and the Cold War began. They exist solely to enforce peace and defend each other when necessary

Hungary-Romania War
Article 5 was enforced after Asgarzchs invaded and occupied Hungary and Romania since both are NATO member, other members involved include Poland, Bulgaria, the UK, Germany, France and the US.

Factions
US Army British Army Bundeswehr French Foreign Legion Polish Land Force Bulgarian Army