In his address to the nation at the start of Operation Iraqi Freedom, George W. Bush states that the threat to the country is being met with the military now so that it doesn't have to be met again with firefighters and first responders on the streets of American cities. What event is he referring to?

Saddam Hussein's use of chemical weapons on the Kurds
the Oklahoma City bombing in 1995
Iraq's invasion that led to Desert Storm
the attack on the World Trade Center in 2001