In Vietnam there was a Ho Chi Minh Trail from a secure (from ground attack) North Vietnam, which had borders with China (secure from any sort of attack). The fighters (mostly North Vietnamese Army) in South Vietnam had a supply line of Chinese and Russian arms. They enjoyed secure refuges in Cambodia and Laos. Johnson and to a lesser extent Nixon forced our commanders to fight the war with one hand tied behind the back.
In Iraq, there are some stockpiles of weapons and ammo from the Saddam days. There is no supply line. There are no secure refuges. The level of intensity we see now is unsustainable. And this time we aren't looking to negotiate with the terrorists.