Federal authorities have always made it difficult to bring a legal challenge against the government’s warrantless wiretapping enterprise that was set up by the Bush administration in the years after the Sept. 11, 2001, attacks. Because the wiretaps were secret, no one could know for certain if they were being tapped, so the government urged judges to throw out lawsuits for lack of proof of real harm.

That strategy was halted on Monday when a federal appeals court said that civil liberties and journalism groups challenging an eavesdropping law could pursue a suit trying to get the government’s wiretapping declared illegal. In an important ruling, the United States Court of Appeals for the Second Circuit reinstated a lawsuit that a federal district judge had thrown out in 2009.

The new decision might lead to a significant — and far too long delayed — legal review of the statute.

The law in question, passed in 2008, amended the Foreign Intelligence Surveillance Act. It essentially legalized retroactively President George W. Bush’s outlaw program of wiretapping certain terror suspects without a warrant. It also immunized telephone companies that cooperated in the program.

And it permitted the government to listen to the international phone calls of Americans who are not engaged in criminal activity, and to read their e-mail messages. At great cost to the privacy of innocent people, it reduced the longstanding protections of judicial supervision over these powers.

The law was challenged by human rights, labor and news media organizations, led by the American Civil Liberties Union. They argued that their communications with clients and interview subjects outside the country would almost certainly be monitored under the law, in part because their jobs required conversations with activists and others whose work would be of interest to the government. Some are lawyers representing accused terror suspects in the United States and often need to communicate with the suspects’ family members or acquaintances outside of the country.

The government argued that the plaintiffs had to prove that they were monitored or harmed, but the Second Circuit didn’t buy that defense. The plaintiffs had every reason to believe that they were being monitored, the court said, and some even spent considerable sums to go abroad for meetings to avoid the eavesdropping.

The final outcome of this legal challenge is far from certain; the government, if it follows its pattern, is likely to cite another familiar defense that a full trial would reveal state secrets. But just by allowing this lawsuit to proceed, the Second Circuit has sent an important message: The government cannot count on simplistic legal arguments to avoid scrutiny of its program to spy on civilians. When one challenge is allowed, others will follow.