Some examples:
Direct:
Funding, arming, and training the Mujahadeen to fight the Soviets. They became Al Qaeda.
Funding and supporting Saudi Arabia, the world's #1 sponsor of terror (and perpetrators of 9/11)
Funding and arming the Syrian rebels against Assad (secular). These rebels were Islamist fighters with large overlap with ISIS. That's where most of the weapons ended up.
Funding Libyan Islamist groups against Gadhafi (secular)
Funding Hamas in the 1980s - this was done by Israel, but Israel is a US client-state and you can be sure we were involved here.
Indirect:
1953 Iranian Coup - admitted to by the CIA -reinstallation of the autocratic Shah and overthrow of Iranian democracy - directly led to the Islamic Revolution 30ish years later and the boon of Islamic Terror. Many consider this the original sin in the current Middle East situation.
2003 Iraq invasion - destabilized the region, put millions of young military aged men out of work and gave them a reason to hate the west (hundreds of thousands of civilian deaths). This became the starting point for ISIS.
Special treatment of Israeli contravention of International Law - strong recruiting tool for Islamist groups. US routinely uses its special veto to stop UN sanctions of Israeli violations of International Law. These resolutions usually have near unanimous consent other than the US and occasionally the UK.