The threat to the survival of which nation led the Allies to pursue a "Europe-first" strategy?

Americans were very negative towards the war and did not want anything to do with it. However when the Japanese started making advances on the pacific coast, the U. S. Declared naval war upon japan. When france fell, and england was constantly being bombarded by the Nazi navy, America thought if they could resecure Europe from the Nazi then they could flank the Japanese. Sorry for rambling your answer is England (U. K., Great Britain)