I seriously cannot comprehend _any_ of the key words here: "oil," "imperialism," "defeat." No one has said the invasion was about oil; it was about _control_ of oil. There seems no common ground on which to debate.
Capitalist corporations (European, U.S., perhaps Russian & Chinese) now handle Iraq's oil. The presence of U.S. troops in Iraq make it impossible for any Middle-Eat power to interfere with that control. Iraq, followed by Libya and Syria, has been destroyed as an independent power. Iran is completely encircled and the capacity of the Iranian government to subsidize consumption has been destroyed.
If that is a defeat for imperialism (freedom for capitalist corporations to operate on a global basis) I would hate to see what a victory would look like.
Carrol