AL UDEID AIR BASE, Qatar — As the United States prepares to intensify airstrikes against the Islamic State in Syria, the Arab allies who with great fanfare sent warplanes on the initial missions there a year ago have largely vanished from the campaign.

The Obama administration heralded the Arab air forces flying side by side with American fighter jets in the campaign’s early days as an important show of solidarity against the Islamic State, also known as ISIS or Daesh. Top commanders like Gen. Lloyd J. Austin III, who oversees operations in Syria and Iraq, still laud the Arab countries’ contributions to the fight. But as the United States enters a critical phase of the war in Syria, ordering Special Operations troops to support rebel forces and sending two dozen attack planes to Turkey, the air campaign has evolved into a largely American effort.

Administration officials had sought to avoid the appearance of another American-dominated war, even as most leaders in the Persian Gulf seem more preoccupied with supporting rebels fighting the government of President Bashar al-Assad of Syria. Now, some of those officials note with resignation, the Arab partners have quietly left the United States to run the bulk of the air war in Syria — not the first time Washington has found allies wanting.

Saudi Arabia and the United Arab Emirates have shifted most of their aircraft to their fight against Iranian-backed Houthi rebels in Yemen. Jordan, reacting to the grisly execution of one of its pilots by the Islamic State, and in a show of solidarity with the Saudis, has also diverted combat flights to Yemen. Jets from Bahrain last struck targets in Syria in February, coalition officials said. Qatar is flying patrols over Syria, but its role has been modest.