President of European Commission Jean-Claude Juncker, left, President of the European Council Donald Tusk, second left, chief of European Foreign Policy Federica Mogherini, second right, and NATO Secretary General Jens Stoltenberg, right, arrive for a EU Signing Ceremony of the EU-NATO Joint Declaration during the NATO summit in Warsaw, Poland, Friday, July 8, 2016.(AP Photo/Czarek Sokolowski)
President of European Commission Jean-Claude Juncker, left, President of the European Council Donald Tusk, second left, chief of European Foreign Policy Federica Mogherini, second right, and NATO Secretary General Jens Stoltenberg, right, arrive for a EU Signing Ceremony of the EU-NATO Joint Declaration during the NATO summit in Warsaw, Poland, Friday, July 8, 2016.(AP Photo/Czarek Sokolowski) Credit: Czarek Sokolowski

WARSAW, Poland — Leaders of the NATO military alliance on Friday began a landmark summit that will order ambitious actions against a daunting array of dangers to the security of their nations and citizenry, including a rearmed and increasingly unfriendly Russia to Europe’s east and violent Islamic extremism to the south.

“As the challenges we face change and evolve, so does NATO,” alliance Secretary-General Jens Stoltenberg said as the summit opened. He said the decisions to be made by U.S. President Barack Obama and the 27 other heads of state and government will “shape NATO for years.”

The trans-Atlantic political and military alliance, founded in 1949, must adapt “so our people are safe, our countries are secure and our values are preserved,” said Stoltenberg.

Polish President Andrzej Duda, the summit’s official host, warned that Western democratic values are being undermined by a “notorious lack of respect for international law” as well as terrorism and new high-tech techniques of warfare, and said NATO needs a coherent strategy to combat those problems.

After arriving in Warsaw, Obama announced the U.S. will send an additional 1,000 U.S. troops to Poland as part of a NATO effort to reinforce its presence on the alliance’s frontiers near Russia.

In an op-ed published in the Financial Times, Obama called on NATO to stand firm against Russia, terrorism and other challenges, and to “summon the political will, and make concrete commitments” to strengthen European cooperation after the British vote in June to leave the European Union.

Stoltenberg said that keeping for NATO’s member nations safe includes supporting partner nations in the Middle East and North Africa menaced by extremist violence.

“For our nations to be safe, it’s not enough to keep our defenses strong, we must help to make our partners stronger,” he told a pre-summit of defense and security experts.

“Training local forces is often our best weapon against violent extremism,” Stoltenberg said. Among the items on the summit agenda is increased assistance for Iraq’s military, extension of the West’s financial commitment to the Afghan military and police, aid for Tunisia, and getting NATO more involved in the campaign against the Islamic State by authorizing use of AWACS surveillance planes to assist the U.S.-led coalition fighting the group.

In Warsaw, NATO heads of state and government will also formally order deployment of multinational units on the alliance’s eastern borders. The action, telegraphed in advance like most items on the summit program following months of deliberations by NATO member governments, is vigorously opposed by the Kremlin. But German Defense Minister Ursula von der Leyen on Friday called it an appropriate measure to counter what she called a “completely unpredictable and aggressive Russia.”