Mathews was always opposed to war and militarism, and campaigned against conscription in the bitter referendums of 1916 and 1917. In 1917, he was one of several political figures prosecuted under the War Precautions Act with having "made false statements of a kind likely to affect the judgment of the electors" in the referendum, but represented himself and was acquitted. Mathews was struggling with serious health issues by the mid-1920s, and retired due to ill health at the 1931 federal election.
Mathews had always been against fighting and military power, and fought against mandatory military service in the intense plebiscites of 1916 and 1917. In 1917, he was among several prominent politicians charged under the War Precautions Act for making false statements that could sway voters' decisions, but represented himself and was found not guilty. Mathews was struggling with serious health issues by the mid-1920s and stepped down due to poor health at the 1931 federal election.