Vietnam War. 1955-1975. Over 58,000 Americans dead. (19 yrs., 5 mo.)
By 1972 when McGovern ran, it had gone on and on for 17 of those years, with about 56,000+ of those already killed.
(Nixon had run in 1968, claiming to have a "secret" way to win the war.)
The American people run out of patience, for "half-measure" wars. We will soon be at 16 yrs, for Afghanistan and yet we still allow hostile locals, to infiltrate our ranks and kill us.
We defeated Germany, Japan on several continents, in less than 4 yrs, because we went "all out."
We had 10 percent of our population in uniform. We killed civilians on the enemy side, as a means of bringing about unconditional surrender, and ending hostilities.
It worked, we won, and those wars have stayed won.