"The only reason why the government decided to proceed with this trial is so that it could pursue this dangerous theory that equates leaks to the press with aiding the enemy," said ACLU spokesman Ben Winzer.
Others saw the acquittal on the main charge as a victory for free speech.
"It shows that a really very junior enlisted person can do battle with the federal government in a case where the government is really mad as hell about what happened here, throws everything it has at him, and its biggest charge fizzles," said Gene Fidell from the National Institute of Military Justice.
During Manning's sentencing hearing, Congress will convene a hearing on the future of the National Security Agency's surveillance programs in the wake of a second major intelligence leak: Edward Snowden's leaks of records to journalist Glenn Greenwald.
Greenwald, who writes for the British daily newspaper The Guardian, believes Manning's convictions are evidence of differential justice, he told CNN's Anderson Cooper on Tuesday.
He said the soldier was just doing the job journalists should do to make government transparent to Americans.
Greenwald compared Manning to famed journalist Bob Woodward, who gained international fame when he broke the iconic Watergate wiretapping scandal. Its cover-up led to the resignation of President Richard Nixon.
"Bob Woodward has written book after book after book and has become extremely rich by publishing secrets way more sensitive than anything Bradley Manning ever published," Greenwald said.
The difference in the eyes of U.S. justice, in Greenwald's opinion: Woodward is well connected with senior officials who leak to him.
Manning, he said, is not.