March unfolded like a stress test for U.S. counterterrorism authorities. The month opened with a gunman in an Iranian-flag shirt killing three people at a bar in Texas. Then, an attack with homemade explosives outside the mayor’s mansion in New York City. Next came a deadly shooting March 12 on a Virginia college campus and, the same afternoon, a car-ramming at a Michigan synagogue. Days later, agents arrested a man charged with threatening a mass shooting at an Ohio mosque. To current and former national security officials, these were omens, signs of the dangers they predicted last year when President Donald Trump began redirecting counterterrorism resources toward his mass deportation campaign. They had warned of a diminished ability to respond should major global events inflame threats at home and abroad. Now, they say, the war in Iran has locked the Trump administration into a showdown with a sophisticated state sponsor of terrorism at a time when U.S. security agencies have hemorrhaged expertise and leadership is in flux. The urgency of the moment has trained a …