Special Forces of the Polish army attack a house during NATO military exercises in June. The force is NATO's response to Russia's annexation of Crimea and the conflict in eastern Ukraine. (Sean Gallup/Getty Images)

The Pentagon is increasingly concerned about how to combat “hybrid warfare,” the combination of stealth invasion, local proxy forces and international propaganda that Russia used to annex Crimea and destabilize eastern Ukraine, U.S. officials said.

Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, on Thursday released the 2015 National Military Strategy, in which he cited Russia’s actions in Ukraine and said “hybrid conflicts” will persist well into the future.

This kind of warfare transcends traditional notions of one military confronting another by incorporating conventional and unconventional forces, information warfare such as propaganda, as well as economic measures to undermine an enemy, according to Frank Hoffman, a professor at the National Defense University.

“The critique was, and still is, that America’s view of war is overly simplified,” he said. “We think of things in black-and-white terms.”

The issue animated Defense Secretary Ashton B. Carter’s recent trip to Europe.

“How do we confront cyberattacks, propaganda campaigns and hybrid warfare?” Carter asked during a speech in Berlin. “How do we ensure we can deal with more than one challenge at a time?”

The newly fashionable term is a relatively old concept; its essential elements had been part of Russia’s and China’s military doctrines long before the Kremlin sent its so-called “little green men” into Crimea, Hoffman said.

“This is something that we have to do better as the United States to identify and deal with,” Rep. Mac Thornberry (R-Tex.), chairman of the House Armed Services Committee, said in an interview. “This poses a challenge for us, and adversaries know that. They’re looking to run between the seams and confuse and delay us.”

Thornberry has included a provision in the 2016 National Defense Authorization Act calling on the Pentagon to develop a strategy to counter hybrid warfare.

“Hopefully, this provision in the bill helps Secretary Carter get more of the thinking and the intellectual heft of the department in helping us have a more effective response,” Thornberry said

Gen. Philip M. Breedlove, NATO’s top military commander and the commander of all U.S. forces in Europe, said that NATO and U.S. Special Operations forces had begun working with countries with substantial Russian minorities, such as Estonia and Latvia, to help them prepare for potential subversion from the east.

“We have groups of people, primarily in our special forces, that help work with nations to help understand those skills and those capabilities and capacities in their nations to address hybrid warfare,” Breedlove said.

The Kremlin on Thursday rejected accusations that it had acted aggressively in Ukraine or had any plans to undermine its neighbors. In a response to the strategy outlined by Dempsey, Dmitry ­Peskov, a spokesman for Russian President Vladi­mir Putin, said it indicated a “confrontational attitude, devoid of any objectivity towards our country.”

While in Europe, Carter announced that the United States would be contributing troops and “enabling capabilities” to NATO’s new Spearhead task force that would include surveillance aircraft and additional Special Operations forces. Carter added that, aside from rapid crisis response, the task force would be augmented to help counter cyberthreats and other dangers.

Hybrid warfare “is one of the dimensions of our adaptations and very important [for] countries surrounding Russia that don’t want to be susceptible to the kind of thing that happened in Crimea,” Carter told reporters.

But Hoffman said NATO is unable to confront hybrid warfare on its own.

“NATO is a military alliance, and the game is being played on a different field,” Hoffman said. ­“Either NATO works with the E.U. or with other people that has these kind of tools.”