Which nation did the allies think posed the most serious long-term threat?

Germany. I hope this helps.

Explanation:

Germany was in focal Europe and had the capacity to pull crude materials from all the vanquished countries encompassing it. It was unrealistic to separate Germany without retaking the nations the Germans had caught. Focused assaults on German rail lines may have helped however there were a great deal of trains in German control and a large number of miles of track, which wasn't difficult to fix.  

The Germans were always updating their gear and there was an immense improvement from 1940 to 1945, particularly in military aircraft and tanks. A Japanese Zero warrior was not an advanced contender by 1944, yet the German FW190 and ME262 were forefront.