Look carefully at media images of ground fighting across the Middle East, and you will notice that the bad guys are fighting differently than they have in the past... We see Islamist fighters becoming skilled soldiers. The thrust of the Islamic State down the Euphrates River illustrates a style of warfare that melds old and new. U.S. soldiers fighting in Iraq used to say: “Thank God they can’t shoot.” Well, now they can. They maneuver in reasonably disciplined formations, often aboard pickup trucks and captured Iraqi Humvees. They employ mortars and rockets in deadly barrages. To be sure, parts of the old terrorist playbook remain: They butcher and execute prisoners to make unambiguously clear the terrible consequences of resistance. They continue to display an eager willingness for death and the media savvy of the “propaganda of the deed.”

... The Darwinian process of wartime immersion has forced them to either get better or die.

What we see in Gaza, Syria and Iraq should serve as a cautionary tale for any Beltway guru calling for a return of U.S. forces to Iraq. U.S. soldiers and Marines are still the global gold standard, but their comparative advantage has diminished. As terrorist groups turn into armies, pairing their fanatical dedication with newly acquired tactical skills, renewed intervention might generate casualties on a new scale — as the Israelis have been painfully learning.