Answer: In the aftermath of the war, internationally the world was changing, Europe was slicing up Africa, many countries started fighting for their independence, and the fight for influence and money ensued between the most powerful nations. During the time periods of 1865-1900, the US sought to keep up with Europe and expand its sphere of influence in the world under the leadership of Roosevelt, McKinley and other presidents.
Internal terrorism and external terrorism both pose a great threat, but terrorism carried out by the United State's own citizens can be much more harmful in the long run. My favorite quote about this is nerdy, but it's from Captain America: Civil War and is stated by the true antagonist of the movie, "An empire toppled by its enemies can rise again. But one which crumbles from within? That's dead... forever." We as a people can rally against an outside threat and rise from whatever they may do to us, but we become exponentially weaker if we must rally against those within our own borders. That's just kinda my opinion.
