Defeating radical jihadists today requires all states to act aggressively against them - an exceptionally difficult challenge. Some states aid terrorist groups by being passive as terrorists operate on their soil. The experiences of Saudi Arabia and Pakistan vis--vis al-Qaeda, as well as the US tolerance of Provisional Irish Republican Army activities during the 1970s and 1980s, are instructive. These cases suggest that passive support usually occurs due to strong popular support for a terrorist group's cause, a perceived lack of a direct threat to the regime, and limited costs to the government for tolerating these activities. Halting passive support is difficult and requires new policies and a new way of thinking about the problem of state support for terrorism.