Five Somali men accused of firing assault rifles at a Navy ship off the coast of Africa are set to face the first U.S. piracy trial in more than 100 years.

The suspected pirates are accused of shooting at the USS Nicholas in an attempt to plunder what they thought was a merchant ship. Instead, they fired on a battle-tested, 453-foot ship patrolling the pirate-infested waters, which shot back, forcing the men to flee in their small skiff, prosecutors said.

The men, along with other suspected pirates, were eventually captured and brought back to the U.S. to stand trial. Yet, until now, no case has actually gone to a jury. The federal trial will begin Tuesday and is expected to last about a month.

The most infamous pirate captured in the spring was Abdiwali Abdiqadir Muse. The Somali suspect who staged a brazen high-seas attack on the U.S.-flagged Maersk Alabama pleaded guilty in New York to charges he hijacked the ship and kidnapped its captain. He faces a minimum of 27 years in prison.

The group of men accused in the USS Nicholas attack April 1 face a much stiffer punishment if convicted of piracy, which carries a mandatory life sentence. Yet the charge may be difficult to prove for prosecutors, in part because the suspected pirates never actually boarded the vessel.

The government acknowledges the five defendants did not take control of the Navy frigate with a crew of 100 highly-trained sailors, which defense attorneys argue is necessary to prosecute the piracy count.

"They fired on a Navy ship. That's the whole case," said David Bouchard, an attorney for the Somali men. "The didn't go on the boat. They didn't shoot anybody. They didn't rob it."

In a similar, but separate case involving a group of alleged pirates who are suspected of firing at the USS Ashland on April 10, a federal judge has thrown out the piracy charge, ruling there was not enough evidence to prove the charge. Prosecutors are appealing the judge's decision.

Prosecutors say an 1820 Supreme Court decision and contemporary international law show that the alleged actions of the Somali nationals constituted piracy.

U.S. District Judge Mark S. Davis has allowed the piracy charge in the USS Nicholas case to go ahead in Norfolk, home to the world's largest naval base and homeport to the Nicholas.

Ken Randall, dean of the University of Alabama School of Law and a piracy law scholar, said the two judges have different views, but he thinks the government's prosecution will ultimately prevail because U.S. vessels were involved.

"From what I've seen, no, I don't think the piracy count is particularly challenging as a matter of law," Randall said. "Because the definition of piracy really has existed along similar lines for three, four centuries, and the alleged conduct seems to clearly fit that definition."

The USS Nicholas piracy trial would be the first in the U.S. in at least a century, according to legal and maritime scholars. Other countries have recently held piracy trials, but one of the last in the U.S. was in 1861, when 13 Southern privateers aboard the Savannah were prosecuted in New York City. The jury deadlocked and the men were later exchanged with the South.

The defendants in the USS Nicholas case are also charged with plundering, firearms counts, assault and other charges.

These cases are part of a larger U.S. policy debate over how best to deal with the insurgents and criminals in Somalia, a poor and barely functioning nation that is suspected of harboring al-Qaida-linked terrorists.

Somalia-based piracy continues despite an international flotilla that has reduced the number of hijackings in the Gulf of Aden and forced pirates farther south. Somalia has not had an effective government for 19 years, one of the reasons piracy has been able to flourish.

Somalia's first secretary to the U.N. mission has called the international prosecution of Somali pirates "vigilante justice."

"I find it difficult to believe that the international community is rendered helpless by a bunch of teenagers, chewing khat (a narcotic), and armed with AK47s and RPGs," Omar Jamal said.