Not quite. The YA was a federal institution, therefore, the troops had been stationed in all six republics since the end of WWII. The reinforcements were sent in only after the TO (Territorial Defence; a Yugoslav equivalent of the U.S. National Guard structure) had attacked the barracks and taken over the external borders of Yugoslavia (the Yugoslav-Italian & the YU-Austrian stretch of the border in the Socialist Republic of Slovenia). The YA acted in accordance with the Yugoslav Constitution ("preservation of the territorial integrity").
The YA was at the disposal of the Federal Executive Council, which, through the Supreme Command (consisting of representatives from all 6 republics), commanded the troops.
Could it be that those countries (Germany & Britain, primarily) did not recognize the legitimacy of Yugoslavia to begin with (after all, it was not a single country until the 20th Century)?