Question

Explain the factors that influenced Americans to play a more active role in the world.

Answer 1

One factor would be how America is a nation built on the premise of democracy and equality for all its citizens, yet established great security for civilians as well. Because of these strong values upheld by our nation, citizens felt it was our duty to help others who did not receive the same equality in their respective countries.
 
Despite the Farewell Address written by George Washington, where he said, "It is our true policy to steer clear of permanent alliances with any portion of the foreign world; so far, I mean, as we are now at liberty to do it; for let me not be understood as capable of patronizing infidelity to existing engagements."

We have come to Ronald Reagan's quote, "We cannot play innocents abroad in a world that is not innocent." 

Answer 2

The most likely reason for this is economic gain. It is a very simple premise that it serves in the interest of the US to have a stable world all around where they can invest in and later also receive greater returns. This is true for the Middle East where more oil is being pumped today from Iraq compared to pre-Sadam times, despite the country being very unstable. But this isn't true only for the Middle East, any nation where a stable country would benefit the US more, is something that the US government considers in their interest. This is also the reason why they started taking a more active role in the world.