There are many possible phrases that could be used, with different phrases to describe the US and various countries in Europe. As some examples, we could use:
- Idealistic and pacificistic. This would describe the US and many European countries. This is why they made treaties like the Kellogg-Briand Pact.
- Economically devastated. This could apply to Germany more than any country since much of its wealth was taken to pay for the damages from the war.
- Full of political unrest. This would apply most to Russia and Germany. However, you could apply it a bit to the US where there was a "Red Scare" in the time immediately after WWI.