Some of the president's own advisers argue the United States does not need the European allies for a war against Iraq. But during the war in Afghanistan, American bases in Germany, Britain and Turkey were crucial for moving troops, supplies and bombs into the theater. And many of the same allies have made major contributions to rebuilding Afghanistan with humanitarian programs and peacekeepers.
Would they do the same if there were a war with Iraq?
"It's shortsighted to say we don't need the Europeans in the war when you forget about the very important contribution they will have to make in the peace," said Daalder.
"The more countries that we have involved, the greater the international support we will have for the reconstruction of Iraq, which is likely to prove to be the longest and most difficult part of this whole process," said Kenneth Pollack of the Council on Foreign Relations.
In public, Pentagon and White House officials say they are confident most allies will back the United States when and if the president makes his case for war. In private, some concede the opposition is much stronger than expected and could cause major delays in making any decision.