“Anger has been building up like hot magma beneath the surface, and the Osprey could be what finally causes an eruption,” said Takeshi Onaga, the mayor of Naha, the Okinawan capital, and a member of the conservative Liberal Democratic Party. “If they force the Osprey onto us, this could lead to a collapse of the U.S.-Japan alliance.”

Of course, opposition to the American bases is nothing new in Okinawa, and it remains unclear how far the protesters would actually go. However, most analysts in Japan and the United States seem to agree that Okinawan anger is reaching levels unseen in recent times. They say this has put the United States in a difficult position. “You cannot let politics dictate what platform you use,” said James Schoff, a former senior adviser on East Asia affairs for the Office of the Secretary of Defense. “But in this environment, an accident is going to be like setting a match to a tinderbox.”

Japanese officials say they have tried to allay Okinawan concerns by conducting their own inquiries into the recent crashes, with the inquiries accepting the Pentagon’s findings that pilot error was to blame. During a visit to Tokyo last month, Defense Secretary Leon E. Panetta signed an agreement to allow the Osprey to fly in Japan with restrictions aimed at ensuring safety.

However, those efforts have failed to appease the island’s deeply rooted anger. With more than half of the 50,000 American military personnel in Japan stationed there, many Okinawans say their island remains a virtual military colony, long after the United States returned it to Japan in 1972. Okinawans say this has led to increased awareness about the discrimination that they say Okinawa has suffered since Japan seized the once-independent kingdom in the 1870s.

The sense of alienated outrage adds to the longstanding anger over Futenma, which has become a symbol of the Japanese failure to ease Okinawa’s burden. Tokyo and Washington still have yet to put into place a 16-year-old deal to relocate the base from Ginowan, which was originally signed in response to the 1995 rape case. Three years ago, frustrations reached a new high when the left-leaning Yukio Hatoyama, then the prime minister of Japan, raised hopes by promising to move the base off Okinawa, only to renege in the face of domestic and American pressure.

Okinawan emotions remain raw at what was seen as Mr. Hatoyama’s betrayal.

Okinawa got a taste of civil disobedience over the weekend, when police officers with riot shields towed more than a dozen vehicles that protesters had used to briefly seal off the Futenma base’s three gates — something opponents say they have not tried before.

“If they impose that dangerous thing on us, then all hell will break loose,” said Satoru Oshiro, 48, a labor union employee who joined a dozen protesters to use two vans to block the base’s Nodake Gate on a recent morning. “Enough is enough.”