Specifically, we either don't define metrics for success or we define completely unrealistic metrics. Worse, we go to war with no intention of destroying an enemy that has left us with no alternative other than to risk many, many human lives to stop them. We go for political wins at home, to claim ground in a geopolitical power struggle, or for natural resources.
*snort*
I agree I'm not sure when a goal of eliminating the enemy completely is one that I could support, though interestingly enough that is precisely what Israel's claimed goal currently is with regards to Hamas. I believe its also the stated goal of Russia, though I'm not familiar with what faction of neo-nazis they claim to be targeting or whether they even exist in any meaningful numbers in Ukraine.
https://en.wikipedia.org/wiki/Operation_Enduring_Freedom
The US lost 2,380 and the coalition killed 72,000+.