Consistently assailed for refusing to stand up to the president, Senate Republicans this week briefly found their voices, bombarding Mr. Trump with public complaints over his Syria decision. The fleeting moment of dissonance revealed what has emerged as an informal rule of thumb among Republican senators who consider themselves foreign policy experts, with wide latitude to weigh in and potentially influence a president who has far less experience on the subject than they do. They are willing to break with Mr. Trump on matters of international affairs — but only when they believe there are no domestic political consequences for doing so…

“Many of us have been dealing with this for a decade or two decades, and I think there are a lot of visits to the area and a lot of discussions that stand behind our views on these issues,” said Senator Roy Blunt, Republican of Missouri, who had previously counseled the White House on the necessity of maintaining forces in Syria. “This is an area where it has been a consistent concern that leaving those places would create bigger problems than staying.”…

“Foreign policy has always been Trump’s Achilles’ heal with Senate Republicans,” said Alex Conant, a Republican strategist and former staff adviser to Senator Marco Rubio of Florida, another Republican who faulted the president’s Syria decision — but not his comments about China and Ukraine. Mr. Conant said Republicans were also driven by their view that Mr. Trump’s foreign policy missteps were more damaging, requiring a more forceful response than his day-to-day incendiary statements.