The 1st Army Group was a Royal Yugoslav Army formation mobilised prior to the German-led Axis invasion of Yugoslavia in April 1941 during World War II. It consisted of the 4th Army, 7th Army, and the 1st Cavalry Division, which was the army group reserve. It was responsible for the defence of northwestern Yugoslavia, with the 4th Army defending the eastern sector along the Yugoslav–Hungarian border, and the 7th Army defending the western sector along the borders with Germany and Italy. Like all Yugoslav formations at the time, the 1st Army Group had serious deficiencies in both mobility and firepower as well as internal friction among the different ethnic groups, particularly between Serbs and Croats.

The 4th Army was a Royal Yugoslav Army formation mobilised prior to the German-led Axis invasion of the Kingdom of Yugoslavia during World War II. It was drawn from the peacetime 4th Army District. When mobilised, it consisted of three divisions, a brigade-strength detachment, one horse cavalry regiment and one independent infantry regiment. It formed part of the 1st Army Group, and was responsible for defending a large section of the Yugoslav–Hungarian border, being deployed behind the Drava river between Varaždin and Slatina. Like all Yugoslav formations at the time, the 4th Army had serious deficiencies in both mobility and firepower.