President Obama referred to the “evolving threat” of terror attacks during his address to the nation Sunday night in the aftermath of the mass shooting in San Bernardino, California. That attack and the Islamic State assault on soft targets in Paris symbolizes the president’s worries. Instead of highly sophisticated acts of terror like the September 11th attacks, now heavily armed individuals detonate explosives or fire at civilians gathered in public spaces. Last week, it was a work holiday party in San Bernardino; in 2013, it occurred during the famed Boston Marathon; four years prior, an army psychiatrist began randomly shooting colleagues at his base in Texas. While Obama’s critics condemn what they say is an incoherent and inadequate response to Islamic State militants, many experts agree there is no simple answer or strategy. The intersection of extremist ideology masking as religion, the availability of weapons, a flood of war-weary immigrants and those who might be inspired by Middle Eastern militants have made the war on terror far more challenging than ever before.