Why did anti communist forces fight in the vietnam war even though they knew they would lose?

Actually I believe it was Johnson who mumbled something about not being the first president to lose a war. What ever. The fact of the matter is the US military handed the politicians two, and arguably three military victory's that should have ended the war in our favor. All three times the politicians failed to recognize the opportunity they were handed (LBJ in 68 and again in 68, and Nixon in 72/73) PI$$ED the victory away. The contention the Vietnam War was unwinable is a myth created by the antiwar movement in the late 60s and promoted by the biased liberal media every since then. It is not accurate.