We fought a military war; our opponents fought a political one. We sought physical attrition; our opponents aimed for our psychological exhaustion. In the process we lost sight of one of the cardinal maxims of guerrilla war: the guerrilla wins if he does not lose. The conventional army loses if it does not win.

Henry Kissinger (1923-2024) German-American diplomat
“The Viet Nam Negotiations,” Foreign Affairs (Jan 1969)
    (Source)

Sometimes paraphrased as "A conventional army loses if it does not win. The guerrilla army wins if he does not lose."