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DUBAI, United Arab Emirates (AP) — The U.S. Navy again has accused Iranian patrol boats of harassing an American warship in the Persian Gulf, this time with a Revolutionary Guard vessel nearly causing a collision with the USS Firebolt. Why does this keep happening? ___ HEAVY US MILITARY PRESENCE The U.S. Navy has had a regular presence in the Persian Gulf since the end of World War II. It ramped up its involvement in the region after Iran's 1979 revolution, which toppled the U.S.-backed shah and alarmed Washington's Gulf Arab allies. The 1991 Gulf War and the 2003 U.S.-led invasion of Iraq brought even more American forces to the region. Today U.S. ships patrol the Gulf and its narrow Strait of Hormuz, through which nearly a third of all oil traded by sea passes. Iran views the U.S. presence as a provocation. ___ PAST CONFRONTATIONS During the Iran-Iraq war in the 1980s, Iran mined portions of the Persian Gulf and several commercial ships sank. In 1988, the USS Samuel B. Roberts struck a mine and nearly sank. That sparked a daylong naval battle between Iran and the U.S. in which American forces attacked two Iranian oil rigs and sank or damaged six Iranian vessels. A few months later, the USS Vincennes in the Strait of Hormuz mistook an Iran Air flight heading to Dubai for an attacking fighter jet, shooting down the plane and killing all 290 people onboard. ___ AFTERMATH OF THE NUCLEAR DEAL The hard-liners who dominate Iran's security forces were largely opposed to the landmark nuclear deal that President Hassan Rouhani, a moderate, reached with the United States and other world powers last year. Police have arrested a number of dual citizens on security-related allegations since the deal was struck, and there has also been an uptick in provocative acts at sea. The U.S. Navy has recorded 31 instances of what it describes as "unsafe and/or unprofessional interactions" with Iranian forces this year alone, compared to 23 in all of 2015. ___ ROCKET LAUNCHES AND SAILOR SEIZURES The provocations cited by the U.S. Navy were largely blamed on Iran's powerful Revolutionary Guard, a paramilitary force charged with protecting clerical rule. Iranian speedboats fired rockets near U.S. warships and commercial traffic in December, and an Iranian drone overflew an American aircraft carrier in January. Iran also briefly seized 10 U.S. sailors in the Gulf at gunpoint in January after their boats drifted into Iranian territorial waters. In Sunday's incident, the Guard's fast-attack boats came within some 450 meters (500 yards) of the USS Firebolt, with one stopping right in front of the coastal patrol boat, said Cmdr. Bill Urban, a spokesman for the U.S. Navy's 5th Fleet, based in Bahrain. Urban said the USS Firebolt turned and missed the boat by only some 90 meters (100 yards). ___ RISKS REMAIN AMONG THE WAVES For the U.S. Navy, each "unsafe" Iranian action "creates a significant risk for our ship's commanding officers, who have mere seconds to decide on appropriate actions to take to defend their ships and crew," Urban said Wednesday. Iran has vowed to continue its patrols, has blown up replicas of U.S. vessels in drills and says it can close the Strait of Hormuz at will. ___ Jon Gambrell is senior Gulf correspondent for The Associated Press. Follow him on Twitter at www.twitter.com/jongambrellap. His work can be found at http://bigstory.ap.org/content/jon-gambrell .

AP Explains: Why do US, Iran often face off in Persian Gulf?