If Russia revives Cold War tensions in the future would the US be able to help defend the EU and UK coalition?

The UN wouldn't be able to do anything without America's approval and isn't really a government (for example it doesn't issue currency), so in the event of Russia and it's satellite nations reviving Soviet tensions again could the European Union and UK (commonwealth allies and nations with the Queen with head of state included, such as even Australia, New Zealand, and Fiji) count on America for military istance?