Actually, Islamic aggression has been around a lot longer than this country has.  Al Qaeda is just one of the recent flavors of it.

The enemy of my enemy is my friend.  We sided with the Northern Alliance (Ne: Al Qaeda) because they served our interest at the time to curb the aggression of the Soviets (ne: Russia), our ultimate arch enemy and competitor for world domination.  Later, it served our interests to side with Saddam Hussein in the Iran-Iraq war because Iran had designs on hegemony of the region and taking out the Saudis in order to control 80% of the world's oil supply, which would give them a grip on our proverbial short hairs.  Then Saddam became a tyrant against his own people and it served our interests to take him out and establish a presence in the center of the chess board.  Obama gave that up, and things in that region have gone downhill since.  It's down to the Sunni and Shia fighting each other to control most of the worlds oil, and both have their extremists.