Question

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

Answer 1

The threat to the survival of the Soviet Union led the Allies to pursue a “Europe-first” strategy.

Answer 2

The threat to the Soviet Union and Britain led the allies to pursue a "Europe First" strategy.

The Europe First strategy was actually a military or war strategy where the allies focused to subdue the Nazi Germany in Europe first. So it is also called Germany First strategy. Only Soviet Union and Britain were not dominated by the Germany in Europe. So Germany was a great threat to their survival which  made them to adopt the German or Europe First Strategy.