Who knows - perhaps one could argue that invasion and occupation of the United States was impossible. But the isolationist case in this day and age is even more ludicrous.
I would argue that al Qaeda can't conquer the US. But they can continue to stage large scale terror attacks against us. Prior to 9/11, we lost just under 100 people to al Qaeda terror attacks. But they were all military personnel. If al Qaeda had confined itself to attacking government and military people on station overseas, neither Afghanistan nor Iraq would have been invaded. The problem is that al Qaeda has become addicted to killing the easiest targets of all - civilians, preferably American ones, to attain their objective of getting us to force Muslim governments to capitulate to them.
Even if we don't go after them, they will never stop going after us - they see us as the key that will open the door to power in the Islamic world. In the al Qaeda worldview*, when the Great Satan withdraws his support, Muslim governments will fall like ripe fruit into al Qaeda's palms. By the logic of al Qaeda's arguments, if they don't fall, why, the Great Satan must not truly have withdrawn his support from those "puppet" Muslim governments. They must therefore continue to punish Uncle Sam with mass death until those governments do fall. This is why al Qaeda is a mortal enemy. It's like a rabid dog that won't let go.
* This idea is ripped off from the Marxists - that all states with any relations with the US were puppet states kept standing only with great difficulty by Uncle Sam. The reality is that most of the non-Communist states stood on their own two feet, whereas the Communist states had to be heavily subsidized by the Soviet Union to avoid deprivation and economic collapse. Just look at North Korea today. Saudi Arabia relies on us to provide protection from Iran, but it doesn't need us to protect it from its internal opponents. It can take care of domestic insurrections just fine.