BAGHDAD — The blast walls and barbed wire came down. The checkpoints went away.

Fifteen years after it was sealed off, the heavily fortified neighborhood in the heart of Baghdad was opened to the public on Monday.

The neighborhood, known as the Green Zone, had been cordoned off by the American military in 2003 to protect it from bombings during the war. The four-square-mile patch of land contained Saddam Hussein’s palaces, which later housed the headquarters of the American occupation authorities and military, and the Parliament building, the seat of the new Iraqi government.

But its history and isolation made it a potent symbol first of the American occupation and later of the alienation felt by many Iraqis toward their own government, and Iraqi leaders have been promising to reopen it ever since the American military withdrew in 2011.

The reopening came on the first anniversary of Iraq’s recapture of its northern city, Mosul, from the Islamic State group, and officials said it would be a trial opening over the coming two weeks. Traffic was allowed on main thoroughfares but many side roads, where officials live and work, remained closed and government buildings like the Parliament and Mr. Hussein’s old Republican Palace remained under heavy guard.