WASHINGTON — Defender 2020 in Europe is set to be one of the largest exercises the continent has seen in decades. And while it will test the U.S. Army’s ability to project capabilities from the continental United States to nations across Europe, the opportunity will also put NATO to the test.

The U.S. and its NATO partners and allies acknowledge none of them will fight alone in a war against an aggressor in Europe, and thus operating jointly is critical but also difficult.

The U.S. Army has several years of experience performing tactical readiness drills at the brigade level in Europe through its gapless rotations of armored brigade combat teams and combat aviation brigades. But with Defender, the service will demonstrate strategic readiness, testing its ability to respond with force and project itself across Europe in coordination with its fellow NATO members and partners, Lt. Gen. J.T. Thomson, the head of NATO Allied Land Command, told Defense News in an interview ahead of the Association of the U.S. Army’s annual conference.

The exercise will test “all the systems that go with that kind of strategic reinforcement,” he said.

Defender 2020 is set to be the third-largest military exercise in Europe since the Cold War, Lt. Gen. Chris Cavoli, the U.S. Army Europe commander, told Defense News in an exclusive interview earlier this month. The division-scaled exercise will test the Army’s ability to deliver a force from “fort in the United States to port in the United States,” and then to ports in Europe, and from there to operational areas throughout the continent, including Germany, Poland, the Baltic states, Nordic countries, and Georgia, among others, Cavoli said.

The exercise will involve at least 15 NATO countries and two partner nations, Thomson said. And NATO specifically will participate at the corps level down to the tactical level, Thompson noted.

“From a land forces standpoint, the demonstration of collective defense is our best deterrent,” he said.

“We’re actually doing collective defense, and I stress collective, just not one or two nations,” he said. “This is from fort to port. This isn’t just a river crossing or a specific fight, it’s very comprehensive in nature.”

While the U.S. puts its National Defense Strategy Multi-Domain Operations concept to the test in Europe, NATO will evaluate its own strategic approach, according to Thomson.

For NATO, its ability to receive forces and equipment from the U.S., stage them, move them forward onto the battlefield and integrate them will be the focus throughout the exercise. Though this has been simulated before, “in this case, we’re not simulating it, we are doing it,” Thomson said. “Once those forces get integrated, we’re actually going to conduct defensive operations collectively.”

Crucial to NATO will be evaluating the current state of military mobility and ensuring countries can seamlessly operate together. But those are also the biggest challenges, Thomson said.

During the Defender exercise, theater mobility will be put to the test at a massive scale — something that hasn’t always been easy.

Defense News flew on a Black Hawk from Bulgaria to Romania during the U.S. Army-led exercise Saber Guardian in 2017 with then-U.S. Army Europe Commander Lt. Gen. Ben Hodges when crew members were alerted they might have to land for an unanticipated customs check. The delay would have caused the general, who was leading the entire exercise, to miss a live-firing demonstration on the Black Sea.

A few emails and phone calls later, the stopover was diverted. But the experience highlighted the red tape the military runs into on a regular basis. Hodges called for the establishment of a “military schengen zone” that would ease border crossings, but that evolved to a focus on “military mobility” across Europe.

Since then, the European Union has worked to improve crossings for militaries and their equipment over the past several years to ensure rapid movement.

But there are still hiccups, Thomson said. During Defender, participants must move massive amounts of equipment and troops across countries in the northeast of the continent. The effort will test infrastructure and border policies. Forces will have to cross through EU member countries and nations that are not part of the organization, such as Norway, and each nation has its own set of rules, policies and procedures.

Ensuring nations can be interoperable has been a challenge for NATO. “I’m fond of saying there’s no such thing as 100 percent interoperability, not even within nations,” Thomson said, but “we are headed in the right direction on interoperability. We test it and train on it daily across NATO” through work with the enhanced forward presence units in Poland, Latvia, Lithuania and Estonia and at other episodic exercises.

“This one is an outstanding opportunity to do it at division, corps and joint task force level,” Thomson said. “We don’t do that that often.”

At the exercise, interoperability will be tested as U.S. Army Europe serves as a combined joint force land component command and a NATO corps operates underneath it alongside American divisions. “The scale of this one will give us very good lessons and some good azimuth to work into the future,” Thomson explained.

From 2020 onward, the Defender exercise will become an annual series taking place in both the Pacific and Europe, but every other year will be a “light” year — referring to the number of participating troops — the acting U.S. Army Pacific commander, Lt. Gen. John “Pete” Johnson, told Defense News in a recent interview. The drill in Europe will be “heavy” this year, and the Pacific version will be smaller. In 2021, the Pacific-based Defender will have its turn being the larger of the two.