legal: 1) of, based on, or concerned with the law; 2) appointed or required by the law. [1]

Military force was authorized by Public Law 107-243 AUTHORIZATION FOR USE OF MILITARY FORCE AGAINST IRAQ RESOLUTION OF 2002 [2].

[1] http://www.merriam-webster.com/dictionary/legal [2] https://www.gpo.gov/fdsys/pkg/PLAW-107publ243/html/PLAW-107p...