WIESBADEN, Germany — Often criticized for being naïve and complacent about Russian President Vladimir Putin’s ambitions, the Obama administration and its European allies are intensifying their planning and training for a potential conflict on NATO’s eastern flank, hoping to send the strongest possible message about Russian aggression.

Deterring Putin was a central theme at a conference here hosted by the U.S. Army, attended by generals from 38 European countries. Officials described escalating NATO activity — including a massive training exercise now underway and talk of U.S. troop increases — in terms reminiscent of the 1980s, when the alliance stared down Soviet-made Warsaw Pact tanks in central Europe. Tan and brown desert camouflage is out; forest greens are in.

The NATO exercise, named Trident Juncture, is the largest in more than a decade; it began in mid-October and wraps up late next week. It involves 36,000 troops responding to the invasion of a fictitious region called Ceresia by a hostile neighbor employing so-called hybrid guerrilla warfare tactics like those Putin has adopted in Ukraine. Operating in Spain, Portugal, Italy and surrounding Atlantic waters, the counter-force includes submarines, fighter jets and U.S. Marines storming beaches with landing craft.

The training is partly designed to increase military readiness — a sign of concern that Putin might launch another rapid military operation like his March 2014 seizure of Crimea, which caught the U.S. and Europe flat-footed.

But perhaps just as important, military leaders said, it helps Moscow understand that the U.S. and its allies are ready and willing to throw a counter-punch should he try something similar again.

“In order to deter an adversary, he has got to realize your capabilities. He’s got to be able to see it and smell it — from the news, from the exercises, from things like this conference,” the U.S. Army’s chief of staff, Gen. Mark A. Milley, told POLITICO. Milley was a co-host of the annual Conference of European Armies, a two-day affair in which generals from places as disparate as Lithuania and Italy discussed strategy, readiness and coordination.

“In order to deter an adversary, he has got to realize your capabilities" — U.S. Army chief of staff

While Washington’s focus has recently shifted to Putin’s Syria intervention, U.S. military planners in Europe are braced for another Russian thrust in their theater, possibly in the Baltic states of Latvia, Lithuania and Estonia. Attendees said that scenario was a key topic amid enduring concerns about the Islamic State and conflict refugees.

Russia’s growing shadow has generals and staff at the U.S. Army headquarters here working longer hours than they have in years, planning in detail for combat scenarios considered obsolete since the fall of the U.S.S.R. “We are busy as hell,” said Col. Bill Williams, deputy chief of staff at the Army headquarters.

Russia was joining U.S.-led military exercises in Europe as recently as two years ago. But Putin’s startling annexation of Crimea and subsequent assault on eastern Ukraine ended that practice. Increasingly, NATO’s exercises are designed with a Russian enemy in mind.

One operation touted at the conference, for instance, was an exercise staged this fall under the moniker of Dragoon Crossing. It involved rapid river crossings in Hungary and Romania, with mechanics laying bridges under simulated artillery fire. Such a scenario could be necessary to rush forces toward a Russian incursion from the east.

That was one of six war games scheduled for October and November alone, with bellicose names like Saber Strike, Iron Sword and Fall Storm.

Amid the military posturing, some U.S. and European officials concede that manpower and readiness shortfalls — a product of Western budget tightening — may limit their deterrent message and tempt Putin to make new territorial claims. Senior officials at U.S. Army headquarters in Europe routinely say their job is to make fewer than 30,000 U.S. troops appear like a force of 300,000.

“We’re supposed to demonstrate strength,” said one U.S. military official based in Europe. “But let’s be honest — we’re not very strong over here.”

Some U.S. and European officials concede that manpower and readiness shortfalls — a product of Western budget tightening.

“Clearly, NATO isn’t as strong as it used to be — or needs to be,” says Ivo Daalder, a former Obama ambassador to the alliance and now president of the Chicago Council on Global Affairs. Daalder said NATO is still stronger than Russia and could defeat any attack by Moscow on its territory.

U.S. and NATO officials are debating the possibility of rotating more American troops into the Baltic states, which now host about 150 American soldiers apiece. Any increase is likely to be modest. But the mere presence of U.S. troops in those countries serves as a tripwire: any Russian assault would likely claim American lives, thereby guaranteeing a severe response from Washington.

In an interview, Milley said the Army was “evaluating” its force structure and would submit recommendations to the Pentagon at an unspecified time. The Pentagon announced plans in June to pre-position about 250 pieces of heavy equipment, including tanks and Bradley fighting vehicles, in six eastern European nations to shorten America’s response time in the event of a conflict with Russia.

Milley said Russia would be mistaken to underestimate the West’s mettle. “The unity and strength of NATO is real,” he said.

But he left open the possibility that he would recommend deploying more troops and equipment to Europe. “We don’t need Cold War-levels of forward-deployed forces or anything like that,” Milley said. “But at the same time, you want to make sure you have sufficient capabilities to deter further aggression by Russia.”

Deterrence requires a tricky balance, however. Officials said they understand the need to demonstrate a willingness to fight — without provoking Putin unnecessarily.

Officials here also face more practical problems. One is a kind of strategic whiplash, in which generals who spent years focused on Islamic insurgencies and terrorist manhunts are dusting off old doctrines of maneuver warfare with mechanized divisions. Gray-haired generals are feeling déjà vu even as they marvel at post-9/11 enlistees with no memory of the Cold War.

Russia enjoys some advantages, including an edge in offensive electronic warfare. (Williams called the Russian capability in that area “amazing.”) The Russians also enjoy the freedom of rapid movement while European countries are debating how to speed up military movements across their borders. Those can be stalled for days, particularly at the boundaries of non-NATO members.

Nor is NATO completely unified. Southern European countries are less worried about the fate of the Baltics than about a growing refugee crisis caused by chaos in the Middle East and northern Africa.

Other nations, including France and Germany, fret that the kind of Western muscle flexing showcased here could undermine their efforts to maintain a fragile cease-fire in eastern Ukraine. This summer, Russia’s foreign ministry warned that one NATO exercise in Ukraine was “a mistake” that could have “explosive consequences.”

Pro-Russian separatists whom U.S. officials say are directed by Moscow continue their rebellion. But Obama officials say Putin has lowered the intensity of the conflict in recent months, likely with an eye toward winning relief from European Union sanctions in early 2016.

Amid it all, senior officials here and in Washington say it’s difficult to divine Putin’s intentions. Some doubt he has a long-term strategy, likening him to a checkers player. Others fear he is outwitting the West in a game of chess — backed up by a greater appetite for military confrontation.

Some fear Putin is outwitting the West in a game of chess — backed up by a greater appetite for military confrontation.

Boosting their anxiety is a recent spate of “snap exercises” by Russian forces, held without advance notice. One such exercise, in March, involved 80,000 Russian personnel. “The scale of this exercise means it could only have been a scenario simulating a war with the United States and/or NATO,” concluded Thomas Frear, an analyst with the London-based European Leadership Network.

U.S. planners are particularly worried about Putin’s intentions toward the Baltic states. All three are all members of NATO, whose charter holds that aggression against any member requires a response by all members.

U.S. military officials worry about two scenarios in particular. One is a replay of Russia’s hybrid warfare in Ukraine. The fear is that Putin might spark an insurgency among the ethnic Russian population of the Baltics, but with deniable tactics — like propaganda, cyberwar and covert operatives — that might not obviously trigger the NATO treaty’s collective self-defense provision.

Another nightmare scenario is a lightning Russian strike into the eastern Baltics executed too quickly for NATO to check it. The resulting Russian foothold would be hard for NATO to dislodge.

U.S. officials firmly insist they will defend the NATO alliance. In a visit to Estonia last September, President Barack Obama vowed to protect the Baltics. “You lost your independence once before,” Obama said. “With NATO, you'll never lose it again."

But the decisive question may be political, not military. Opinion polls, which the generals here say they have noted, show skepticism within key European countries about fighting for the tiny Baltics. Nearly 60 percent of Germans oppose doing so.

“Most NATO publics surveyed are reluctant to live up to the promise in Article 5 of the North Atlantic Treaty that member countries will assist allies who are attacked, including with armed force,” the Pew Research Center found in June.

Even so, Daalder insisted that recent events have sent Putin an unmistakable message. “Those exercises and beefing up of our forces and reinforcement capabilities will have convinced Putin NATO is very serious about defending every inch of its territory,” he said.