Whether we started them or not is mostly irrelevant. We've joined and extended plenty of wars, and given 9/11 I'd say we didn't "start" Iraq and Afghanistan out of the blue. We just didn't know exactly who we were supposed to be fighting.

According to this [1] the US has been in ~117 wars (if I counted right) in 238 years.

[1] http://en.wikipedia.org/wiki/List_of_wars_involving_the_Unit...