• December 5 – The Army Corps of Engineers took over jurisdiction of surplus military airfields at Glasgow, Lewistown, and Cut Bank.
• December 15 – The U.S. Sixth Army invaded Mindoro. A train wreck at Gold Creek killed one man and burned up a freight car full of cigarettes.
• December 16 – A German offensive began in the Ardennes Forest.
• December 22 – The U.S. 101st Airborne held Bastogne against repeated German attacks.
• December 23 – Fire destroyed the McBee Motor Company garage in Billings with $100,000 damage. A man died of a heart attack while helping move belongings from an apartment adjacent to the fire.
• December 24 – The Charlo butter and cheese factory was destroyed by fire with $25,000 damage.
• December 26 – The Fourth Armored Division relieved the 101st Airborne at Bastogne.
• December 27 – Governor Sam Ford was sworn into his second term.
• December 28 – The Bachelor Officer’s Quarters at Gore Field was gutted by fire.
• December 31 – Longtime Missoula mailman Louis Brutto retired after 40 years on the job. Beginning in 1918, he had walked an estimated 100,000 miles delivering the mail.