Patiently.  Over decades.  With containment, and, when possible, constructive engagement.  Even as hotheads wanted us to risk nuclear annihilation over Korea, over China, over Vietnam, and over Cuba, we had faith that ours was the superior ideology. We were correct.
Oh, you mean Herr Hitler.  Long after Hitler had overrun Western Europe and was knocking on England's door, and even after his ally Japan attacked us, we did not declare war on Germany. 
Read your history.  Germany declared war on us, not vice versa.
There is always a bogeyman out there, be it the Nazis or the commies or ZOMGSaddamHussien or now, the Mooselums.  And there are always those short-sighted cowards amongst us who are quick, in their quivering fear, to abandon our bedrock ideals while they urge Armaggedon, be it the scumbag scourge of McCarthyism then or those pandering politicians now saying we shouldn't take in any of the refugees fleeing ISIS terror because one or two might be moles.
Fear-based abandonment of our highest, most inclusive ideals -- give us your poor, your tired, yearning to be free -- is the reactionaries go to response, and always has been.
But it is our most tolerant, most inclusive ideals which are, and always have been, our highest, best strength, and our saving grace.  It is those ideals, not always lived up to but still held as our goal, which have enabled us to prevail over the more hateful ideologies.
ISIS promotes fear, hate, death and exclusion.  Our own right wing panderers promote their own brand of this in reaction.  It is up to the rest of us not to let either prevail.