ANKARA, Turkey (AP) - U.S. President Donald Trump and his NATO counterparts are gathering in Turkey on Tuesday for a two-day summit that comes at a turning point in the organization´s history as the United States steps back from its traditional security role in Europe.

Ahead of the meeting in Ankara, Trump has insisted on "loyalty" after some NATO countries balked at allowing U.S. forces to use their bases for attacks on Iran. He listed big European members Britain, France, Germany, Italy and Spain for criticism.

A NATO summit is a highly symbolic moment when the 32 member countries of the world´s biggest military alliance underline their unshakeable commitment to each other´s security. This year, though, the trans-Atlantic bond has rarely seemed more fragile.

Still, the meeting is being organized around the theme of a stronger Europe in a stronger NATO. The Trump administration has called for a reboot to a "NATO 3.0," and it´s hoped that what this really means will become clearer over the next two days.

President Recep Tayyip Erdogan is hosting the summit at his vast Bestepe Presidential Compound on the western edge of the Turkish capital, Ankara. A new airport, converted from an old military airfield, has been unveiled especially to host NATO leaders.

Security will be high. Air defenses are on alert, and tens of thousands of police will be on duty. Nearby neighborhoods are closed to traffic and some state workers have been given time off to help keep roads unclogged. Public gatherings are banned.

More than a dozen people were detained in security sweeps ahead of the summit, including two journalists, the Turkish Journalists Association said.

On Tuesday evening, Erdogan will host a dinner in his "Winter Garden." Top officials from Japan, South Korea, Australia and New Zealand will join their NATO partners. Ukrainian President Volodymyr Zelenskyy is scheduled to attend.

At they dinner, foreign ministers will hold a NATO Ukraine Council, while the alliance's defense ministers will meet with their Indo-Pacific counterparts. A separate meeting with officials from Gulf Arab countries will also take place, and Trump will meet with Syrian President Ahmad al-Sharaa.

NATO leaders alone will hold a single working session on Wednesday morning. They´ll publish a short statement summarizing the results of their meeting once it´s over.

Officially topping the agenda is defense spending - a perennial issue at NATO as the U.S. presses allies to do more. Ahead of the summit, NATO Secretary-General Mark Rutte noted a 20% annual spending hike by European allies and Canada in 2025.

This is unlikely to be enough to satisfy the Trump administration, even after the leaders agreed at their last summit to boost investment to the same level as the United States, in gross domestic product terms. The 2026 U.S. military budget is set at $901 billion, or about 3.3% of GDP.

NATO also wants to highlight the way it´s converting the billions pouring in from state coffers into new military kit that´s adapted to modern warfare. The summit will be a chance for the organization to showcase new military projects.

A defense industry forum will be held on the sidelines of the meeting, on Tuesday, bringing senior NATO and partner officials together with industry leaders, as allies push to ramp up weapons production and spur innovation in new technologies.

Another top agenda item is continued support to Ukraine, now in a fifth year of full-scale war with Russia. European allies and Canada are funding most of Ukraine´s needs, including paying for about 90% of the country´s air defenses.

The working session is only expected to last about three hours, but most debate is likely to focus on U.S. force levels in Europe and the off-agenda item of fallout from the U.S.-Israel war on Iran.

European allies and Canada will want reassurances, or at least clarity, on U.S. force intentions. Since early last year, they have often been blindsided - and sometimes confounded - by Trump´s declarations on cutting troop numbers.

Ahead of the summit, the Pentagon surprised the allies by announcing a 6-month review of the U.S. presence. It´s focused on progress Europe makes to defend itself, but also on whether the U.S. has full base access and overflight.

NATO played no active role in the Iran war and has no overarching agreement with the United States on the shared use of bases and airspace, although some of its members do.

At a public meeting with Rutte on June 24, Trump renewed his criticism of the allies for their reluctance to get involved in the war. "We don´t need their money - we don´t need anything," he said. "I just want loyalty."

On joining NATO, member countries pledge loyalty equally to each other through a commitment to collective security - the all-for-one, one-for-all pledge enshrined in Article 5 of NATO´s treaty. That guarantee alone underpins everything the organization stands for and does.

What further loyalty Trump might require is unclear.