WASHINGTON — Democrats were thrilled when John Walsh of Montana was appointed to the United States Senate in February. A decorated veteran of the Iraq war and former adjutant general of his state’s National Guard, Mr. Walsh offered the Democratic Party something it frequently lacks: a seasoned military man.

On the campaign trail this year, Mr. Walsh, 53, has made his military service a main selling point. Still wearing his hair close-cropped, he notes he was targeted for killing by Iraqi militants and says his time in uniform informs his views on a range of issues.

But one of the highest-profile credentials of Mr. Walsh’s 33-year military career appears to have been improperly attained. An examination of the final paper required for Mr. Walsh’s master’s degree from the United States Army War College indicates the senator appropriated at least a quarter of his thesis on American Middle East policy from other authors’ works, with no attribution.

Mr. Walsh completed the paper, what the War College calls a “strategy research project,” to earn his degree in 2007, when he was 46. The sources of the material he presents as his own include academic papers, policy journal essays and books that are almost all available online.