Senior officials say the smaller Army presence will focus on sustaining ties with allies through an enhanced series of training rotations and war-game exercises, mostly based in the region of Grafenwöhr, Germany.

“We are optimistic about increasing those opportunities for forces to train and exercise with our allies,” said Lt. Gen. Mark P. Hertling, commander of United States Army forces in Europe. “Preparing for global employment in unified land operations, strengthening alliances and building partner capacity are consistent with our mission.”

Even senior European officials acknowledge that it is in the best interests of both the United States and the European allies for Washington to reshape its approach to their territory and adapt NATO to the new realities of the 21st century.

The Obama decision on reducing ground forces in Europe “was not a surprise,” said one senior French official. “If some American troops leave Europe, it won’t create any political problems,” he added. “We don’t need a massive presence of U.S. troops. After all, we don’t see Russia anymore as an enemy or an adversary, but even as a partner, if a difficult one.”

A second senior French official expressed skepticism that European nations would be prompted to spend more in their own defense, as they should do, he said, because “no one feels a threat from inside Europe.” But he also cautioned that only two other NATO nations — France and Britain — had the ability to project substantial military power in defense of alliance interests. “Together we are 70 percent of the projection capacity of Europe,” the official said, “and we’re both cutting back.”

The Obama administration will make the case that the American military can position itself in ways that enhance security across the Continent, where threats may come as much from terrorism and cyberwarfare as from military attack.

Thus, while the Army withdraws two brigades from Germany, the Navy will add four Aegis-class warships to Rota, Spain, as part of a Europewide system to defend against potential missile attack from Iran. In other new deployments for that European missile-defense system, which is a part of NATO’s strategic planning, the United States will build a radar system in Turkey and plant missile interceptors in Romania and Poland.