As the arrangements at Al Taqqadum show, even deploying small teams of advisers at a new base can involve much greater troop commitments. The number of Americans actually involved in advising the Iraqis at the base would be just a small fraction of the 450 announced by the administration.

While American officials said earlier this week that 110 would be directly involved in training and advising, on Thursday they said there would be just 50 advisers. They will be split into two teams, Special Forces who will work with Sunni tribes, and advisers who will work with the 8th Iraqi Army Division. The rest are to provide support, logistics and force protection.

Although the goal is to retake the city of Ramadi, which fell to the Islamic State last month, General Dempsey indicated that an effort may be months away. He said it would take several weeks for the initial command and control center to be set up at Al Taqqadum.

“Timetables are fragile,” he said. “They are dependent on so many different factors.”

For the Pentagon, the timing has been a difficult issue as the United States Central Command and the Iraqi government have clashed over the pace of efforts by Iraqi security forces to retake areas captured by the Islamic State, also known as ISIS or ISIL. While the American military once forecast recapturing Mosul this spring, the fall of Ramadi less than 70 miles from Baghdad put that city much higher on the priority list.

General Dempsey said the United States was still hoping the Iraqi government would find a way to engage Sunnis to beat back the Islamic State, but he also talked of what he called a “Plan B” in case that never happens.