WASHINGTON — Fired by one American commander in chief for insubordination, Michael Flynn has now delivered his resignation to another.

President Donald Trump had been weighing the fate of his national security adviser, a hard-charging, feather-ruffling retired lieutenant general who just three weeks into the new administration had put himself in the center of a controversy. Flynn resigned late Monday.

At issue was Flynn’s contact with Moscow’s ambassador to the United States. Flynn and Ambassador Sergey Kislyak appear to have discussed U.S. sanctions late last year, raising questions about whether he was freelancing on foreign policy while President Barack Obama was still in office and whether he misled Trump officials about the calls.

The center of a storm is a familiar place for Flynn. His military career ended when Obama dismissed him as defense intelligence chief. Flynn claimed he was pushed out for holding tougher views than the Obama administration about Islamic extremism. But a former senior U.S. official who worked with Flynn said the firing was for insubordination, after the Army lieutenant general failed to follow guidance from superiors.

Once out of government, he disappeared into the murky world of mid-level defense contractors and international influence peddlers. He shocked his former colleagues a little more than a year later by appearing at a Moscow banquet headlined by Russian President Vladimir Putin. Given a second chance by Trump, Flynn, a lifelong if apolitical Democrat, became a trusted and eager confidant of the Republican candidate, joining anti-Hillary Clinton campaign chants of “Lock Her Up” and tweeting that “Fear of Muslims is RATIONAL.”

As national security adviser, Flynn required no Senate confirmation vote or public vetting of his record, and his tenure was brief but turbulent.

The Washington Post and other U.S. newspapers, citing current and former U.S. officials, reported last week that Flynn made explicit references to U.S. sanctions on Russia in conversations with Kislyak. One of the calls took place on Dec. 29, the day Obama announced new penalties against Russia’s top intelligence agencies over allegations they meddled in the U.S. election process to help Trump win.

While it’s not unusual for incoming administrations to have discussions with foreign governments before taking office, the repeated contacts just as the U.S. was pulling the trigger on sanctions suggests Trump’s team might have helped shape Russia’s response. They also contradicted denials about such discussions of the sanctions by several Trump administration officials, including Vice President Mike Pence.