The U.S. Army's chief of staff on Tuesday issued a stern warning to potential threats such as Russia and vowed the service will defeat any foe in ground combat.

"The strategic resolve of our nation, the United States, is being challenged and our alliances tested in ways that we haven't faced in many, many decades," Army Chief of Staff Gen. Mark Milley told an audience at the Association of the United States Army's annual meeting in Washington, D.C.

"I want to be clear to those who wish to do us harm … the United States military -- despite all of our challenges, despite our [operational] tempo, despite everything we have been doing -- we will stop you and we will beat you harder than you have ever been beaten before. Make no mistake about that."

Milley's comments come during an election year in which voters will decide a new president and commander in chief -- and a period of increased military activity of near-peer competitors, including Russia and China.

The Army has struggled to rebuild its readiness after more than a decade of extended combat operations in Iraq and Afghanistan. The service has significantly cut the size of its force since the Cold War and decreased its modernization budget in the last decade, Milley said.

"While we focused on the counter-terrorist fight, other countries -- Russia, Iran, China, North Korea -- went to school on us," he said. "They studied our doctrine, our tactics, our equipment, our organization, our training, our leadership. And, in turn, they revised their own doctrines, and they are rapidly modernizing their military today to avoid our strengths in hopes of defeating us at some point in the future."

Milley also quoted a senior Russian official as saying publicly, "The established world order is undergoing a foundational shake-up" and that "Russia can now fight a conventional war in Europe and win."

The general warned that future warfare with a near-peer adversary will "be highly lethal, unlike anything our Army has experienced at least since World War II."

"Our formations will likely have to be small; we will have to move constantly," he said. "On the future battlefield, if you stay in one place for longer than two or three hours, you will be dead."

Despite the challenges, Milley said the Army will adapt to survive such a dangerous battlefield.

"It's a tall order for sure -- to project power into contested theaters, fight in highly populated urban areas, to survive and win on intensely lethal and distributed battlefields and to create leaders and soldiers who can prevail. Tough? Yes. But impossible? Absolutely not," Milley said.

"Make no mistake about it, we can now and we will … retain the capability to rapidly deploy," he said, "and we will destroy any enemy anywhere, any time."

-- Matthew Cox can be reached at matthew.cox@military.com.