"There is nothing illegal about providing military training, and fighting in a foreign military conflict is not per se illegal," George C. Harris, one of Mr. Lindh's lawyers, wrote.
"Central to First Amendment freedoms is the right to associate with unpopular and disfavored groups," Mr. Harris wrote.
Today's filings in the Federal District Court for the Eastern District of Virginia, combined with filings on Monday and more expected Thursday, contain the defense's most detailed arguments to date in trying to get the case dismissed. The trial is set to begin Aug. 26.
The Second Amendment argument was in a footnote in today's dense legal motions. Experts said that this case appeared to be among the first to rely on the Justice Department's newly stated view that an individual has a right to bear arms.
In a case in February, the Justice Department said it believed the Second Amendment, which refers to "a well regulated militia" and "the right of the people to keep and bear arms," covers an individual's right to keep firearms, whether or not that person is in a militia. The Supreme Court has not ruled on the case.
Mr. Lindh's lawyers quoted from the government's brief today, saying the Second Amendment "broadly protects the rights of individuals, including persons who are not members of any militia or engaged in active military service or training, to possess and bear their own firearms."
Mr. Lindh's lawyers said he was protected from any firearms charges for two reasons. One is the individual right to possess firearms. The second is the principle of "combat immunity," which they argued in papers filed on Monday.
Under that principle, the defense argued that Mr. Lindh's participation in a foreign army as a foot soldier was not illegal because all soldiers in combat are immune from prosecution. They said today that "combat immunity" also protected him from firearms charges.