It is well documented that Americans are ever more divided: along party, ideological, socio-economic and cultural lines; by geographic, demographic, racial and religious differences. Our political polarization hampers our ability to tackle important national issues, whether immigration, infrastructure, timely budgeting or closing Guantánamo. The recent government shutdown and the looming threat of another underscore this problem.

The same policy stagnation afflicts our ability to confront the most pressing threats to our security, from North Korea to the risk of terrorists acquiring weapons of mass destruction, from pandemic disease to Russian aggression. Our ability to counter such outside menaces is increasingly undermined by our collective failure to work together. Indeed, the most significant, long-term threat to our security may be our domestic political polarization.

America’s adversaries exploit the vulnerability created by our dysfunctional democracy. Today, in contrast to Sept. 11, a terrorist attack is more likely to divide than unite us, as we saw after Benghazi, San Bernardino, Calif., and, most recently, Niger. This makes us an even more attractive target, as our enemies benefit not only from the initial attack but also from the lasting consequences of a more fractious, fragmented America.

Similarly, the Iranians know that our resolve to prevent them from acquiring a nuclear weapon may crumble under partisan pressure. China is pursuing its economic and strategic ambitions in Asia unconstrained by an America so divided that we jettisoned the Trans-Pacific Partnership agreement we negotiated, while its signatories reap its rewards without us.