WASHINGTON — Nearly four times as many Sunni Islamic militants are operating around the world today as on Sept. 11, 2001, despite nearly two decades of American-led campaigns to combat Al Qaeda and the Islamic State, a new independent study concludes.

That amounts to as many as 230,000 Salafi jihadist fighters in nearly 70 countries, with the largest numbers in Syria, Afghanistan and Pakistan, according to the study by the Center for Strategic and International Studies, a Washington think tank.

The report’s conclusions, drawing on multiple databases dating to 1980 to compile one of the most extensive studies of its kind, underscore the resiliency of these terrorist groups, and the policy failures by the United States and its allies in responding. The findings also highlight the continuing potency of the groups’ ideology and social-media branding in raising money and attracting new recruits as they pivot from battlefield defeats in strongholds like Iraq and Syria to direct guerrilla-style attacks there and in other hot spots.

“Some of these groups do want to target Americans overseas and at home, particularly the Islamic State and Al Qaeda,” said Seth G. Jones, the director of the center’s transnational threats project and one of the report’s six authors. “All this indicates that terrorism is alive and well, and that Americans should be concerned.”