U.S. homeland security and homeland defense activities are converging. Perhaps nowhere more so than in counterterrorism.

America rebuilt its national security architecture after 9/11. Since then, it has institutionalized interagency collaboration, and it has developed proficient techniques that disrupt terrorist plots and attacks further upstream. More recently, the country has rebalanced counterterrorism budgets toward other security priorities. This comes as the shifting threat environment – including the terrorist designation of cartels and transnational criminal organizations – is imposing new strains on a lean legacy architecture.

A war may end with a military withdrawal, but its weapons can continue fighting long after the soldiers have gone. Five years after the United States withdrew from Afghanistan, American-supplied rifles, night-vision devices, and other military equipment have appeared in the hands of militants attacking Pakistan. Weapons intended to strengthen Afghan security forces are now improving the capabilities of armed groups operating across the border. America's longest war ended in 2021, but its security consequences did not.

Washington cannot reverse its withdrawal, but it can address the weapons proliferation that followed. Tracing American-origin arms, strengthening counterterrorism intelligence cooperation with Pakistan, and disrupting trafficking networks offer a practical response. These measures would not require another military intervention. They would help contain a threat that affects regional stability and American counterterrorism interests.