BOSTON (AP) -- Thousands of sailors aboard naval ships from around the world are gathering in Boston to mark the 200th anniversary of the War of 1812 and the Star-Spangled Banner - an event that coincides with this year's Boston Harborfest, a Fourth of July festival showcasing the city's maritime and colonial heritage.

The War of 1812 marked the first time that the United States was threatened on its own soil. The conflict inspired Francis Scott Key to write the first edition of "The Star-Spangled Banner."

The fleet of at least 18 naval ships converging in Boston is part of Operation Sail, or OpSail 2012, and includes sailors and Marines from the United States, Brazil, Ecuador, Colombia, Indonesia, Germany, Norway, Denmark, Canada and England. The vessels are expected to bring 10,000 to 12,000 extra sailors as the Boston Navy Week's War of 1812 bicentennial celebrations travel up the East Coast to New England's largest city, U.S. Coast Guard Chief Petty Officer Jeff Hall said Friday.

"Harborfest by itself is a large event, but we are coupling OpSail with that and the bicentennial of the War of 1812," Hall said. "This is the largest event probably the city has seen since the last big OpSail event back in 2002."

The oldest commissioned U.S. warship, the USS Constitution, is playing a central role in the Boston celebrations two centuries after its crew demonstrated the Navy's superior tactical, gunnery and seamanship talents against British naval forces. The ship's dramatic victories during ship-to-ship battles against what was then the world's most dominant naval force inspired and rallied Americans around their troops and country.

The War of 1812 is very significant because "it established the United States as a world power ... as a force to be reckoned with in the world," said Frank Neely, a spokesman for the USS Constitution, also called Old Ironsides.

Sailors assigned to USS Constitution on Thursday kicked off their participation by serving as the color guard detail for the opening ceremony at Faneuil Hall. They also will also perform gun drills like those done in the War of 1812 era and 18th-century boarding pike drills daily near the Charlestown Navy Yard. The drills will demonstrate to visitors how sailors prepared and fought in battle at sea during that time.

On Wednesday, the Constitution is scheduled to get underway for its annual July 4th turnaround cruise. After the three-hour outing, sailors will serve as the color guard detail for the Independence Day Boston Pops celebration.

The U.S. Navy says the bicentennial celebration is a tribute to all sailors and marines who fought gallantly in the War of 1812 or who have fought in other conflicts since then.

That includes members of the Coast Guard whose crew participated in the War of 1812, including the first-known prisoners of war in Coast Guard history.

"The thing that we're also trying to do by having these celebrations is we're trying to educate the public and inform them that not much has actually changed in the way of the Navy's mission today as it was 200 years ago, I mean we are still fighting pirates," Neely said. "We are still protecting the freedom of the seas - you like your iPod, you like your MP3 player, your TV? That stuff comes, most likely, by way of sea. 90 percent of the world's commerce is traded by sea."