None of those nations really participated in fights on the Western Front, Italy, or North Africa, where US troops were in action.

Thus, the only real combat between the US and these countries was by air attack. The Romanian oil fields at Ploesti were considered an important strategic target and were bombed by the Americans, and defended by a mix of German and Romanian flak + fighter planes. The US Army Air Force suffered its heaviest bomber casualties of the entire war over Ploesti. On August 1st, 1943, the US sent 162 B24 Liberators to Ploesti and over 100 of them were destroyed or damaged during the raid, with a loss of about 500 skilled aircrew. Comparatively the Romanians and Germans lost 7 fighters and the damage to the refinery was negligible. This was the largest single action between the US and the "Balkan Axis". The Americans also bombed Hungary and Bulgaria but there was never any significant ground action between the US and any of these countries.

​

Romania, Bulgaria, and Hungary mostly contributed to the Axis war effort in action against Yugoslavia, Greece, and the USSR, as well as in anti-partisan fighting.