Yes, preferably with a scorched-earth policy.
Were the American lives lost in the Korean War in vain?
I honestly don't know enough about that war to pontificate in any meaningful way.
WW2?
Absolutely. There's no question we were fighting pure evil on both fronts, and we weren't playing nice with either side like we do now.
Do the tactics used or not used by our leaders determine the righteousness of the strategic cause?
Do the ends justify the means? No. Go in to destroy the enemy. Use diplomacy and rebuilding organizations where necessary, realize when/if they can be used (hint: it won't work in Muslim countries).
Tough to scorch Afghanistan :=} but I certainly agree that when we make war it should be done as violently and quickly as possible. Stay safe.