A destroyed North Vietnamese Army tank. The NVA responded to US firepower by switching nimbly from conventional offensives to guerrilla tactics — as the so-called Islamic State seems to be doing now.

US aircraft are flying “50 to 60” sorties a day over Iraq, from food drops to airstrikes, but their impact is local and “very temporary,” the Pentagon’s director of operations told reporters this afternoon. While Lt. Gen. William Mayville didn’t say so outright, it’s clear the majority of missions are still “intelligence, surveillance, and reconnaissance” (ISR) as the US struggles to figure out what’s going on in a fluidly savage situation in which the adversary is adapting nimbly to our actions.

“In the immediate areas where we’ve focused our strikes, we’ve had a very temporary effect,” said Lt. Gen. Mayville, director of the operations (J-3) for the joint staff. That has “blunted” some tactical offensives by the self-proclaimed Islamic State — aka the Islamic State in Iraq and Syria (ISIS), aka the Islamic State in Iraq and the Levant (ISIL) — and it has bought “a little more time” for Yazidi refugees trapped on Mount Sinjar, he told a Pentagon press conference. ISIS forces that were moving confidently in the open have dispersed to “hide amongst the people.” Kurdish peshmerga troops have rallied and driven ISIS back from their regional capital at Erbil (Irbil). The Iraqi central government in Baghdad has even selected a new prime minister, Haider al-Abadi, without (so far) the feared coup attempt by despised ex-premier Maliki.

But the US effort hardly amounts to “breaking the momentum” of the extremists, Mayville made clear, and he fully expects them to regroup and find new weak points to attack. “They’re very well organized, very well equipped; they coordinate their operations [and] have shown the ability to attack on multiple axes,” Lt. Gen. Mayville said.

ISIS’s adaptability and resilience are disturbing traits of what military analysts consider an emerging breed of “hybrid” adversary. The modern template for this theory was the Hezbollah militia in 2006, when surprisingly determined, well-trained, and well-armed irregulars bloodied the vaunted Israelis in southern Lebanon. But the reality behind the “hybrid” theory goes back at least to the Vietnamese Communists, who skillfully blended Viet Cong and regular North Vietnamese Army forces, guerrilla tactics and pitched battle: They massed their forces to invade South Vietnam, dispersed into the jungle after the US intervened, then took Saigon with tank columns once the US was gone. Their inspiration, in turn, was the Chinese Communists led by Mao Zedong, who laid out an explicit progression from political subversion to guerrilla warfare to conventional capture of territory.

What’s particularly critical for such forces – and what was lost on many of Mao’s lesser imitators – is the importance of being able to step back from large-scale offensives to dispersed hit-and-run tactics if the enemy suddenly gets stronger. As ideologically and culturally alien as Sunni Arab zealots are from East Asian Communists, it seems ISIS has grasped the universal applicable principle. That’s not good news.