I'd say, there are many ways to answer this. The best way is to use wikipedia and other web pages similar to that.
Answer:
World War II. This event marked the end of American isolationism and neutralism and the beginning of foreign and defense policy of intense internationalism. ... The U.S. economy was the only major economy that came out of WW II intact. This made the country an instant economic superpower.
The best known of the engravings depicting the Boston Massacre was made by Paul Revere in 1770, but several other versions appeared in Massachusetts and London over the next two years. Each of these images was made to express outrage at the actions of the British troops and to solicit support for the Patriot cause. The images of the confrontation between the soldiers and the townspeople are significant and compelling, but are historically inaccurate. The artists influenced public opinion by depicting a line of Redcoats firing point-blank into a defenseless crowd, when in fact there was no such organized military action and the civilians were an unruly mob of sixty.
THATS ALL I KNOW!
Because the first was in 1803
I'd have to say the westward expansion. mainly because cheap land, economic opportunities
