WASHINGTON — On Monday, Michael T. Flynn resigned as President Trump’s national security adviser after admitting he had misled Vice President Mike Pence about his phone calls with a Russian diplomat. In his resignation letter, Mr. Flynn “sincerely apologized” to Mr. Pence and Mr. Trump, blaming the fast pace of events for what he called his “incomplete” account. Mr. Flynn has a well-established history with Russia, but there is still much about the case that is not clear.

What We Know

■ In December 2015, Mr. Flynn, then retired from the Army, attended a dinner to celebrate the 10th anniversary of RT, a Russian state-backed English-language news channel that disseminates Russian views on global affairs. He sat at a table with President Vladimir V. Putin, spoke beforehand and subsequently appeared on RT as a commentator.

■ Mr. Flynn had multiple conversations with Russia’s ambassador to the United States, Sergey I. Kislyak, before and after Mr. Trump’s November victory. He spoke to the envoy after President Barack Obama announced new sanctions against Russia to punish it for interfering in the American election. Mr. Putin did not retaliate for the sanctions, which surprised Obama administration officials but drew praise from Mr. Trump on Twitter: “I always knew he was very smart!”