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.