Answer:
1. 1919 thru 1940.
2. 1975 thru 2010.
The early portion of American history was dominated by the Indian Wars (1622-1890).
Since 1975, the United States has not fought any NATION exceeding 60 days. They've all been "quick and dirty" (or quick and clean) military campaigns.
Law Enforcement (Police Actions) are the new order. The war on crime (the war on terrorism).