Pre 9/11 we weren't engaged in any foreign conflicts... bored terrorists with nothing to do at home come to America and kill American civilians.
Post 9/11 the military is in their countries... terrorists occupied fighting the US military on foreign soil... ZERO ATTACKS ON US CIVILIANS IN AMERICA POST 9/11.
The only problem is that there is absolutely no proof that these two things are related. And even if they are, is it ok to invade another country which consists of 99,999999% average citizens to keep the 0.0000001% terrorists busy ?
It's like having a talisman to keep away burglars. There haven't been any in your house for 5 years, so it obviously must be working.
Apart from that, what is the strategy, keep the terrorists busy the next 50 years ?
Attacking Iraq imo did not achieve anything, other than to anger the people in Iraq and drive more volunteers towards the terrorist. What does keep the US safe is better screening of passengers (but even there are ridiculous safety measures mixed with sensible ones with no second thought).