Codacons said that the court appears to have considered that former Italian Prime Minister Romano Prodi's government approved the project verbally without providing formal agreement in writing, ANSA, the Italian news agency, reported.
The Dal Molin Air Base in Vicenza houses part of the 173rd Airborne Brigade. The U.S. military hoped to add enough barracks to house 2,100 soldiers, enough for the entire brigade. Part of the unit is based in Germany.
Codacons argued that the government didn't consult local residents.
''We have yet to read the exact motivations behind the court's decision but it would appear they have accepted our arguments in full in regard to the complaints of the local population over the effect the expansion would have had on the environment and the city itself,'' Carlo Rienzi, the group's president, told ANSA.
Vicenza, about 50 miles northwest of Venice, is also home to the base that is headquarters of the Southern European Task Force.