There is a growing threat of peer competitors ready to challenge the U.S., and a need to maintain readiness and interoperability among NATO allies, the commander of Marine Corps Forces Europe and Africa warned Friday.

He was speaking to a gathering of allied amphibious maritime leaders at the Amphibious Leader Expeditionary Symposium held at Rand headquarters in Arlington, Virginia.

“There are peer competitors that are out there that are willing and capable to hand it to us,” said Maj. Gen. Russell A. Sanborn, the commander of Marine Corps Forces Europe and Africa. “Are we ready?”

The illegal Russian annexation of Crimea from Ukraine in 2014 has pole vaulted Russia back into the top spot of major threats to the NATO alliance. And Russia’s investment in long-range cruise missiles poses a grave challenge to U.S. and NATO maritime dominance in the Mediterranean, Baltic and Black Seas.

Paul Schwartz, an expert on the Russian military at the Center for Naval Analysis, said, “Since 2014, Russia has clearly emerged as NATO’s most significant state-based security challenge.”

To meet the challenge, the U.S. and its allies must maintain a credible deterrent threat to keep Russia in check, Sanborn said.

The European theater has 27 amphibious ships, but they don’t always act in sync. This means Russia can go after individual countries, hitting amphibious capabilities one by one, picking rivals off, Sanborn warned.

“We are a gang,” he said of the NATO alliance. “If you mess with one of us, our treaty says if you mess with one, you mess with all.”

The combined effort of Europe’s amphibious ships in the region is a force “that we can all bring to bear,” he said. “Think about the power of being able to do that.”

“Just having the capability is a deterrent.”

The U.S. has spent the past 17 years engaged in low-tech conflicts and conducting operations in relatively easily accessible permissive environments. The challenges America faces in the future will not be so easy. The unabating conflicts have also sapped resources and placed a strain on U.S. readiness.

There’s a myth that “America has the biggest budget,” and that it can do anything that it wants unilaterally, Sanborn said. “That is not reality ― nobody can do it alone.”

Russian provocations over the past several years have NATO and the U.S. on edge as the powerful alliance watches Russia making major geopolitical and strategic movements that threaten European and NATO hegemony in the region.

Russia’s thawing of relations with Egypt has given the Kremlin a foothold into North Africa, where it has propped up rogue Libyan general Khalifa Haftar’s efforts to destabilize the UN- and U.S-backed Government of National Accord.

Moreover, the annexation of Crimea “fundamentally altered the balance of power in the Black Sea in Russia’s favor,” Schwartz said.

And Russia’s intervention in Syria has made it into a “major geopolitical player in the Middle East,” he said.

“Are we ready as a partner, as a coalition, as a gang, to go after that?”