He puts quotes around 'long peace', and links to his definition - https://en.wikipedia.org/wiki/Long_Peace. Notice all the disclaimers there - "absence of major wars between the great powers of the period" ... "period of 'relative peace' has been compared to the relatively-long" ... "wars have declined since the 1950s" "Periods of regional and relative peace" etc. etc.
Also, the record of America's post-WWII wars actually supports his thesis. Except when brighter U.S. Presidents had a "rush in, accomplish very limited objectives, rush out" game plan - those minor wars have ~all proven too expensive to continue. ~Nothing actually gets conquered, and on a military-prowess-per-dollar basis, the American armed forces come out looking pretty underwhelming.
Also, the record of America's post-WWII wars actually supports his thesis. Except when brighter U.S. Presidents had a "rush in, accomplish very limited objectives, rush out" game plan - those minor wars have ~all proven too expensive to continue. ~Nothing actually gets conquered, and on a military-prowess-per-dollar basis, the American armed forces come out looking pretty underwhelming.