The adjustments to the new threats are wide ranging. Hundreds of desert-tan battle tanks and armored fighting vehicles must be repainted dark green to blend into European terrain. Soldiers accustomed to operating from large, secure bases in Iraq and Afghanistan must now practice using camouflage netting to disguise their positions and dispersing into smaller groups to avoid sophisticated surveillance drones that could direct rocket or missile attacks against personnel or command posts.

American troops no longer have unfettered right of way in the air or priority access on the ground, as they did across Iraqi river valleys and Afghan mountain ranges. In today’s Europe, borders count in all matters military. On a recent Friday, an American Army supply convoy rushing ammunition from Germany to Romania was held up at the Austrian border until the next Monday by restrictions on military convoys during busy summer vacation travel periods.

A 10-day exercise last month involving 25,000 American and allied forces spread across three former Warsaw Pact countries — Hungary, Romania and Bulgaria — offered a window into how a generation of senior Army commanders are rehearsing updated tactics and strategies once used to counter Soviet troops, tanks and artillery, including nighttime aerial assaults by hundreds of paratroopers.

To be sure, commanders are expected to argue that the wars in Iraq and Afghanistan will still require thousands of American troops for the foreseeable future. But the United States and its NATO allies recently completed positioning about 4,500 soldiers in the three Baltic States and Poland, and are preparing to keep several thousand armored troops on the Continent as a deterrent to Russian aggression.

These tensions are part of an expanding rivalry and military buildup, with echoes of the Cold War, between Washington and Moscow.

Moscow is flowing forces for its own exercises along its western border with Europe and also deploying in Syria and eastern Ukraine, and is building up its nuclear arsenal and cyberwarfare prowess in what American military officials say is an attempt to prove its relevance after years of economic decline and retrenchment.

In response, the Pentagon has stepped up training rotations and exercises on the territory of newer NATO allies in the east, such as this base in Bulgaria. The allies have increased air, sea and underwater patrols from the Baltic Sea to the Black Sea to counter a similar increase by Russian forces around NATO’s periphery.

The Russian military threat has changed markedly since the Soviet Union collapsed in 1991. President Vladimir V. Putin of Russia has invested heavily in modern infantry forces, tanks and artillery. Moscow has also increased its galaxy of surveillance drones that can identify targets and coordinate strikes launched from other weapons.

Russia’s so-called hybrid warfare combines conventional military might with the ability to manipulate events using a mix of subterfuge, cyberattacks and information warfare. Earlier this year, for instance, Lithuanian prosecutors opened a criminal investigation into a false report of rape by German soldiers stationed in Lithuania as part of a NATO mission to deter Russia. Moscow denied being involved in any disinformation campaign aimed at discrediting allied troops.

With that in mind, top American planners and intelligence officials are closely watching Russian operations in Crimea, eastern Ukraine, and Syria, all proving grounds for new Russian tactics and weaponry. Young American Army officers are once again using flash cards — or the digital equivalent — to study the structure and abilities of Russian Army units, just as American officers did with earlier generations of Russian forces and weaponry in the 1970s and 1980s.

The Army’s training centers in California, Louisiana and Germany are now including more scenarios that replicate Russian forces, even if scenario planners there, and here, are careful to give the opposing forces fictitious names to avoid ruffling diplomatic feathers even more between Washington and Moscow.

The United States Army’s presence in Europe is a far cry from the height of the Cold War, 30,000 soldiers now compared with 300,000 then, General Hodges said. For that reason, the general is putting heavy emphasis on the “speed of assembly” — how quickly troops and their equipment can move hundreds of miles and be prepared to fight at a moment’s notice.

The $40 million exercise here, called Saber Guardian, the largest in Europe this year, included driving more than 1,000 troops and hundreds of vehicles about 1,200 miles across Europe, the equivalent of going from St. Louis to Miami. Hundreds of allied troops — including American soldiers with faces painted in green and black camouflage — and their 60-ton tanks crossed the Danube River on temporary bridges, fending off mock attacks on the other side.

US troops failed to finish the secret mission and all surrounding states are now in course of action.