To doubters of its strategy for defeating the Islamic State, the Obama administration likes to tout its coalition of 66 nations and claim strength in numbers. But a year and a half into the war, some administration officials are acknowledging that this supposed source of strength has its own weaknesses.

Defense Secretary Ash Carter offered a glimpse of his own apparent frustration last week when he referred to "our so-called coalition" and suggested the slackers need to step up.

"We need everybody, and that's all the Europeans, the (Persian) Gulf states ... Turkey, which is right there on the border. So there are a lot that need to make more contributions," he said.

So Carter is leaving Tuesday for Brussels, where he will convene a meeting Thursday of defense chiefs from about two dozen coalition countries, including most NATO members, Iraq and the Gulf states. He indicated he will, behind closed doors, share with them details of the U.S. strategy for recapturing the main Islamic State group strongholds in Syria and Iraq.

"What I'm going to do is sit down and say, here is the campaign plan ... If you're thinking World War II newsreel pictures, you think of an arrow going north to take Mosul and another arrow coming south to take Raqqa," he said last week. And then he will run through a list of military capabilities — "boots on the ground, airplanes in the air," plus trainers and other support personnel — that will be needed to achieve victory.

"And I'm going to say, 'OK, guys. Let's match up what is needed to win with what you have, and kind of give everybody the opportunity to make an assignment for themselves,'" he said. "The United States will lead this and we're determined, but other people have to do their part because civilization has to fight for itself."

A few coalition countries have made gestures of increased support in recent days. The Netherlands, which has been carrying out airstrikes in Iraq, said on Jan. 29 that it would expand its efforts to Syria. Saudi Arabia indicated last week it could send ground troops into Syria, although it was not clear whether the offer was conditioned on U.S. ground forces participating.

Over the course of a decade and a half of coalition warfare in Iraq and Afghanistan, U.S. officials have frequently found themselves pleading and cajoling with the Europeans to contribute more, and they generally have responded with pledges to do just a little bit more. The pattern may be repeated in Brussels. Inevitably it falls to the U.S. military, with greater resources and a longer reach, to carry the biggest burden in countering terrorism.

The air campaign in Syria and Iraq, for example, is advertised as a 13-nation undertaking. But of the 10,060 strikes conducted over the past year and a half — 6,723 in Iraq and 3,337 in Syria, as of Feb. 1 — U.S. warplanes have conducted all but 2,124 of the Iraq hits and all but 208 in Syria. At their low point last August, the allies conducted only two strikes in Syria while the U.S. conducted 210, according to figures provided by the Pentagon. More recently, non-U.S. airstrikes have increased as a share of total strikes.

The U.S. also is counting on another kind of partnership, one that is central to its strategy for not just defeating the Islamic State group but ensuring it stays defeated. It is trying to empower local armies in both Iraq and Syria by partnering with them for training, equipping and advising — without doing the fighting for them. Carter wants more European allies to chip in with trainers and other forms of support.

This cooperation between local and coalition forces is beginning to pay dividends, especially in Iraq, where U.S.-trained Iraqi security forces recently recaptured the city of Ramadi. Iraqi Kurdish forces, known as the peshmerga, have made even more substantial battlefield gains with a great deal of help from U.S. forces.

The Kurdish equation is complicated, however, by sensitivities involving Turkey, a NATO ally that fears a Kurdish insurgency within its own borders. In a report this month for the private Institute for the Study of War, analysts Patrick Martin and Christopher Kozak wrote that the U.S. and its coalition partners may have become over-reliant on the Kurds, and that this threatens to drive Turkey away from deeper cooperation against the Islamic State militants.

"Second, the U.S. risks fueling long-term ethnic conflict in both Iraq and Syria due to the relative empowerment of the Kurds at the expense of other local powerbrokers, often Sunni Arabs," they wrote.