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“That’s nothing, but it’s something, too,” Mr. Weitz said, since it serves the diplomatic goal of showing broad support for Afghan stability.

Afghanistan has said it will contribute $500 million toward its own army. The goal is $2.3 billion from the U.S. and nations outside the fighting coalition, and $1.3 billion from coalition nations other than the U.S.

“You’ll see a strong commitment from allies and partners, and from the Afghan government” in Chicago, NATO spokeswoman Oana Lungescu said.

The White House said Mr. Obama discussed continued support for Afghan forces during pre-summit phone calls Tuesday with the leaders of Australia and Italy.

Britain already has pledged $110 million annually beginning in 2015, and on Wednesday Australia announced that it will contribute $100 million annually for three years.

Afghanistan will dominate the agenda for the Chicago meeting, although there is likely to be little discussion of the military campaign itself. Mr. Karzai is attending, and this week NATO invited Pakistani President Asif Ali Zardari, who has announced he will attend.

NATO is eager to bring forces home but is pledged to a calendar agreed the last time the leaders met, in 2010. Under that agreement, NATO forces will remain in Afghanistan into 2014 and depart that year.

In Chicago, Mr. Obama and other NATO leaders will sign up anew to that schedule, even though a majority of Europeans and Americans now tell pollsters the war is not worth fighting and should end as quickly as possible. In Afghanistan this month, Mr. Obama said the war must end “responsibly,” which cannot mean suddenly.

U.S. and other NATO officials have said there will be no new announcement of troop withdrawals during the Chicago conference. Largely because of public opposition to the war, NATO nations quietly tweaked the 2014 plan earlier this year. The overall deadline holds, but U.S. and other allied forces will shift into largely noncombat roles next year.

The Chicago summit  once was viewed as a possible showcase for progress toward peace talks and a political settlement between Mr. Karzai’s government and the Taliban. There is no real gain to show, however. The insurgents walked away from U.S.-led talks in March. U.S.-backed peace initiatives to open a Taliban political office and transfer Taliban prisoners from the military prison at Guantanamo Bay, Cuba, are in limbo. Insurgents have assassinated the leader and a top lieutenant of the Afghan peace council.