The Vietnam war was started in order to prevent the spread of Communism (as in the domino effect)in the region.

In Vietnam the US originally supported Ho Chi Minh. They provided military assistance during WWII for them to fight against the Japanese.

The US lost that one only after many lives were given up and years passed by. Now it is the turn of the middle east. In Vietnam it was over the control of communism, in the middle east it is over the control of oil and gas.

The Russians tried to influence the area with their war in Afganistan, in the end they too had to pull out, as they could not win a war against irregular troops. The US supplied the Taliban with arms to prosecute that war. Now they have turned against the US.

The US (and the UK) supplied assistance to Iraq in it's war against Iran because then Iran was the greater apparent threat with it's Islamic revolution. Then Saddam turned.

Why do our leaders never learn from history?

Very few Governments have ever won a war against irregular or guerilla armies.