Yes, I should not forget how no one blinked when Hitler made modest advances to restore Germany to its original borders. Little by little, he went unopposed taking all he could grab. And, as previously mentioned above, no country was too eager to launch into more conflict and war with WWI still fresh in their minds.
You can’t discount the uncertainty of the moment even without other factors. Look at Putin and the moves Russia is making now. If Russia launches a war in another few years future people will be screaming how obvious it was that he should have been stopped earlier. Yet, we don’t know. Is he happy now? Will he try to take the Baltic countries? No one knows. Full stop.
The OP asked for short answers when there aren’t any. It was a complex situation and many,many factors came into play.
You made it far too short by rolling Bulgaria and Rumania together as if they were one nation; so much as to be incorrect. Bulgaria has strong historical ties to Russia, it owes its existence as a separate nation rather than being part of the Ottoman Empire to Russia as a result of the Russo-Turkish War of 1877–78. Bulgaria never fought the Soviet Union and was only technically at war with the USSR for 3 days. Bulgarian territory was used by the Germans to invade Yugoslavia and Greece in 1941 and Bulgaria declared war on both, but Bulgarian troops did not participate in the invasion of either. After they fell, Bulgarian troops participated in the occupation of both nations. Bulgaria did not take part in the German invasion of or war with the USSR, Bulgaria neither declared war on the USSR nor sent troops to the Eastern Front. On December 13, 1941 Bulgaria was forced by Germany into making a token declaration of war against both the UK and the United States.