WARSAW — NATO leaders geared up Friday for a long-term standoff with Russia, ordering multinational troops to Poland and the three Baltic states as Moscow moves forward with its own plans to station two new divisions along its western borders.

President Obama said the United States will send about 1,000 troops to Poland, as one of the four NATO battalions being rotated through the region. US defense officials said the troops should be there by the middle of next year, as part of the broader deterrent to Russia.

NATO Secretary-General Jens Stoltenberg outlined several other new alliance actions on the first day of a landmark two-day summit.

He said Obama and leaders of the 27 other NATO countries also took steps toward activating a ballistic missile defense system, recognized cyberspace as a domain for alliance operations, committed to boosting civil preparedness, and renewed a pledge to spend a minimum of 2 percent of their national incomes on defense.

‘‘We have just taken decisions to deliver 21st-century deterrence and defense in the face of 21st-century challenges,’’ Stoltenberg said at a news conference. He said the new NATO units being sent to Poland, Estonia, Latvia, and Lithuania on a rotational basis would stay there indefinitely.

‘‘It’s an open-ended commitment and will last as long as necessary,’’ he said. ‘‘And it is a new reality because we didn’t have that kind of presence in the eastern part of the alliance before.’’

He announced plans as well for an enhanced NATO presence in the Black Sea region, where Russia has also reasserted its influence, with creation of a brigade under Romanian and Bulgarian command.

The secretary-general said keeping members of NATO safe also means supporting partner nations in the Middle East and North Africa that are menaced by extremist violence.

‘‘It’s not enough to keep our defenses strong; we must help to make our partners stronger,’’ Stoltenberg said. ‘‘Training local forces is often our best weapon against violent extremism,’’ he said.

President Andrzej Duda of Poland, 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 high-tech warfare, and said NATO needs a coherent strategy to address those problems.

After arriving in Warsaw, Obama announced his decision to send more troops to Poland as part of the NATO effort to reinforce its presence on the Russian frontiers.

In a column published in the Financial Times, Obama, who is making what is expected to be his last trip to Europe as president, called on NATO to stand firm against Russia, terrorism and other challenges, and to “make concrete commitments’’ to strengthen European cooperation after Britain voted June 23 to leave the European Union.

Creation of the new NATO units is vigorously opposed by the Kremlin.

It follows a raft of other decisions made during the last 22 months to increase NATO’s ability to face Russia and other new security challenges, including tripling the size of the alliance Response Force to 40,000 and formation of a highly nimble Spearhead Force that can start to move within days.

As Obama and the other heads of state and government were gathering in the Polish capital, a spokesman for President Vladimir Putin of Russia said Moscow is willing to cooperate with NATO, even though he said it acts toward Russia like an enemy.

Russia ‘‘has always been open for dialogue’’ with NATO, especially to combat what it sees as a ‘‘genuine threat’’ — terrorism, spokesman Dmitry Peskov said.