In a letter left on Inauguration Day for President Trump that was both congratulatory and cautionary, Barack Obama urged the incoming president to guard democratic institutions and traditions, and “sustain the international order.”

“This is a unique office, without a clear blueprint for success, so I don’t know that any advice from me will be particularly helpful,” Mr. Obama wrote in the letter, which CNN published on Sunday. “Still, let me offer a few reflections from the past 8 years.”

CNN reported the letter was written in longhand on White House stationery and left in the top drawer of the Resolute Desk in the Oval Office. The letter highlighted the concerns of a departing president who acknowledged the enormous responsibility of the job domestically and on the world stage.

Mr. Obama wrote Mr. Trump, a billionaire who had family wealth and made fortunes in real estate, that “we’ve both been blessed, in different ways, with great good fortune,” adding, “Not everyone is so lucky.”