In 1918, Major Shearer of the 5th Marine Regiment, led Marines in the World War I Battle of Belleau Wood in France, where he was awarded the Navy Cross, Distinguished Service Cross, Navy Distinguished Service Medal, Silver Star, French Legion of Honor[3] (Chevalier) and Croix de Guerre.

[9] During World War I, forces under Major Shearer's command made an attack on Belleau Wood, finally clearing that forest of Germans.

[12] His duty in Washington ended in September 1923, when he was attached to the 1st Marine Brigade and sailed to Haiti in order to support government forces against rebels.

Shearer served in the Caribbean until September 1925 and after two-years tour of duty in the United States, he sailed to Nicaragua as a member of the 5th Marine Regiment in June 1927.

During World War II, Shearer remained in active service with the rank of colonel and served as commanding officer of the Marine barracks within Mare Island Naval Shipyard, California.