That the attackers were Saudi citizens is not relevant. They were operating out of Afghanistan. Bin Laden was in Afghanistan. President Clinton had sent a strike team into Afghanistan to get him. Reportedly, they had him in their sights, but President Clinton would not give them authorization to fire, so that came to nothing. He did fire a missile salvo at bin Laden’s training camps in Afghanistan, but he warned the Pakistani government in advance, and he knew that government leaked like a sieve. Bin Laden was warned in advance, and cleared his men out before the missiles struck, so that effort didn’t mean much.
Bin Laden was in Afghanistan after the 9-11 attacks. The Bush administration demanded they hand him over, but Afghanistan declared bin Laden to be under their protection. The 9-11 attacks were technically an act of war by Afghanistan against the United States, but, if Afghanistan had repudiated them, and handed bin Laden over as a criminal, their responsibility would have been considered null. When Afghanistan declared him under her protection, she also assumed responsibility for the attack on the United States. That was an assumption of a state of war. Once Afghanistan did that, President Bush had the legal right to do essentially anything he chose to do against Afghanistan.
Bin Laden fled to Pakistan later, after we went into Afghanistan.