Working under a constant cloud of innuendo that his days in the West Wing were numbered, Gen. McMaster through the year positioned himself as one of Mr. Trump’s most hawkish allies in fractured debates on the president’s top national-security challenges, including North Korea.

But that, in turn, has put Gen. McMaster at odds with other members of the national-security team, especially Secretary of State Rex Tillerson and Defense Secretary Jim Mattis, who have pushed for more measured approaches in policy debates, according to current and former Trump administration officials.

Now, Gen. McMaster is working with strained alliances both inside and outside the White House that have restricted his ability to craft national-security strategy in the Trump White House, the current and former officials said.

Hobbled in his ability to translate Mr. Trump’s sometimes-unorthodox ideas into concepts acceptable to more cautious members of the national-security team, Gen. McMaster has been thrust into a precarious position, with persistent speculation he will be pushed out of his post as soon as the Pentagon finds a suitable new job for him or the White House settles on someone to take his place.

The military is actively looking for a new job for Gen. McMaster, but it could take time to find a suitable position, according to U.S. officials. And the search has been made more difficult in part by his advocacy on behalf of the president’s views and actions. The White House is knocking back speculation that Gen. McMaster will be replaced sooner rather than later. “It’s not a great fit,” said one U.S. official, “but it fits for now.”

While Gen. McMaster may have shallow support in some corners of the administration, he has won grudging praise even from some critics for bringing a sense of order to the National Security Council after the tumultuous monthlong tenure of his predecessor, Mike Flynn, the retired Army general pushed out for misleading the White House about his talks with the Russian ambassador to the U.S.

Allies of Gen. McMaster say he is doing an effective job at exploring and providing a range of policy options to the president, and then helping Mr. Trump to implement that policy, they say. ...