When they attacked on September 11, 2001 they figure that President Bush would fight back
Key part of OBL's master plan, to defeat both Russia and the United States in mountain warfare with the help of batshit-crazy Afghan tribesmen (Rudyard Kipling's Pathans).
First he had to get us up in the mountains, in his chosen battlefield. Watching the World Trade Center go down did the trick.
His objective, shared by the Iranian mullahs, was to delegitimize and displace the Saudis as the guardians of the holy cities of Mecca and Medina. That's the Grand Prize.