A U.S. Army AH-64 Apache flies over the San Gregorio training area near Zaragoza, Spain, during NATO's Trident Juncture exercise, Wednesday, Nov. 4, 2015. In early 2017, U.S. efforts on NATO's eastern edge are set to intensify with an Army tank brigade and a supply of attack helicopters bound for Poland and other nations in Russia's periphery. It is unclear if a President Trump will proceed with the plan or pull back as part of a new arrangement with Moscow.

STUTTGART, Germany — NATO’s top official on Wednesday congratulated President elect-Donald J. Trump on his Election Day victory, but Trump’s rise to the White House is likely to deliver a jolt of uncertainty to Europe and an alliance that for half a century has relied on unflinching American military support dating to the early days of the Cold War.

European officials and analysts said part of the uncertainty stemmed from not really knowing what Trump thinks or plans and urged him to acknowledge America’s responsibilities abroad.

“We face a challenging new security environment, including hybrid warfare, cyber attacks, the threat of terrorism. U.S. leadership is as important as ever,” NATO Secretary-General Jens Stoltenberg said in a statement. “A strong NATO is good for the United States, and good for Europe.”

What isn’t clear is if Trump — who has been highly critical of NATO and European allies he says are not investing enough in defense — will see it the same way.

In Europe, a Trump presidency raises the prospect of a sharp shift in military priorities.

In the wake of Russia’s 2014 intervention in Ukraine, the U.S. military has been a steady presence in the western section of the country, helping to train Ukrainian troops as well as providing nonlethal military aid.

Trump has questioned U.S. interests in Ukraine, which have come at the cost of damaging relations with Russia.

In the Baltics, the U.S. military has been at the forefront of an extensive effort at reassuring allies nervous about a more assertive Russia on their borders. Although Moscow has repeatedly denied it presents any threat to the alliance, U.S. ground forces and air power have been present nonstop for the past few years in Latvia, Lithuania, Estonia and Poland, the linchpin of a strategy that has centered on boosting military readiness.

“We’re entering completely unknown territory,” Estonia’s National Defense Committee chairman Marko Mihkelson said on national television Wednesday. He said he doesn’t expect a fundamental change in U.S. and Estonian relations, but that there are areas of concern.

“Trump has indeed made several statements in matters of security. He has been reserved in his criticism of Russia, especially where Ukraine and Syria are concerned. The question is, what his policy is really going to be?” Mihkelson said.

Germany’s defense minister, Ursula von der Leyen, speaking on national television Wednesday, responded coolly to the prospect of a Trump presidency, voicing concerns about the implications for NATO.

“It was a huge shock,” von der Leyen said when asked about her reaction to the news that Trump was winning. The first challenge, she said, will be figuring out is plans.

“We Europeans, especially as members of NATO know that Donald Trump as president, of course, will ask, ‘what are you contributing to the alliance?’ But we also will ask, ‘what is your stance toward the alliance?’”

Jan Techau, director of the Richard C. Holbrooke Forum for the Study of Diplomacy and Governance at the American Academy in Berlin, said the effect of a Trump presidency on conditions in Europe depends on whether the business mogul believes what he says regarding U.S. security commitments being a matter that is up for debate.

“We don’t know what he actually really believes, and this is the only hope that I have, that he doesn’t believe any of it,” Techau said. “If he believes what he says, we are in for a rough ride. It is a catastrophic problem for the West.”

While a U.S. less inclined toward NATO could spur Europe to stand closer together, Techau said he wasn’t optimistic. Traditionally, the U.S. has been the spur that has encouraged Europe into action on the security stage, and without Washington’s leadership old European rivalries could flare. And with the rise of far right parties in Europe, notably in France and to a lesser degree in Germany, Trump could be a harbinger, Techau said.

U.S. moves in eastern Europe have enraged Moscow, resulting in threats of countermeasures in a region on edge. Trump, who touted the idea of warmer relations with Russian President Vladimir Putin as a centerpiece of his foreign policy outlook, could be inclined to negotiate NATO’s stronger U.S. force posture on Russia’s borders.

Shashank Joshi, a security analyst with the London-based Royal United Services Institute, said that if Trump takes a more conciliatory stance toward Moscow, Europe would likely follow suit, making the prospect of continued sanctions against Russia for its intervention in Ukraine more unlikely.

More military spending on the Continent, particularly in Europe’s east, could also be on the horizon, as nations unsure of U.S. security commitments seek to beef up capabilities. On the other hand, some in Europe also could seek to hedge their bets, aligning more closely with Moscow, Joshi said.

“There is a big risk in seeing a perceived weakening of these security guarantees, which could shift the whole psychological balance,” Joshi said.

While NATO allies in the east have feared the prospect of a more aggressive Russia, Joshi was skeptical that a Trump presidency would embolden Putin to act with force in Europe.

“I think Russia is a savvy operator and there is not a point in becoming aggressive if the politics are shifting in your favor,” he said.

In early 2017, U.S. efforts on NATO’s eastern edge are set to intensify with an Army tank brigade bound for Poland and other nations along Russia’s periphery. A supply of attack helicopters also is headed that way. It is unclear if Trump would proceed with the plan or pull back as part of a new arrangement with Moscow.

When it comes to national security and the role of the U.S. military around the world, Trump will have a freer hand in a way he won’t when it comes to domestic policy. The decision to shut down overseas bases and missions or redeploy forces to the U.S., does not require Congressional approval.

For example, a U.S. missile defense plan in Europe — an area of fierce disagreement between Washington and Moscow — could get a second look or find its way onto the negotiating table as the U.S.-Russia relationship enters into uncharted territory under Trump.

Putin on Wednesday hailed Trump’s election, saying in a message to Trump that he hoped for closer cooperation in dealing with the “burning issues that are currently on the international agenda, and search for effective responses to the challenges of the global security,” RIA Novosti reported.

Retired Adm. James Stavridis, former NATO supreme allied commander and dean of the Fletcher School at Tufts University, said a better sense of Trump’s intentions will emerge once the players of his national security team become known.

“What President-elect Trump has talked about is shaking up NATO and putting it in a pay-to-play basis. In Iraq and Syria, the message included a secret plan to destroy the Islamic State,” Stavridis said in an email. “Few details are available in either idea, and a key indicator of direction will emerge with his choice of defense, intelligence, and cyber advisors and office holders. We will learn much more in the days ahead, hopefully.”

Stars and Stripes reporter Slobodan Lekic contributed to this report.

vandiver.john@stripes.com

