The First Bulgarian Infantry Division was formed in 1935 from the First Infantry Brigade as part of the general program of rearmament in violation of the Treaty of Neuilly. The home station of the division was Sofia. This unit was considered elite and was nick-named the "Iron Division".

It was composed of the 1st, 6th and 25th Regiment of infantry and the 4th Regiment of artillery. The 16th Infantry Regiment was also attached for a time. It was mobilized for the Balkan campaign in 1941, but saw no action.

After a short tour in Yugoslavia, the unit was sent to the Turkish border until late 1942. At least some portion of the division, probably just a reinforced battle group, but possibly the entire division, was on anti-partisan duty in Macedonia -- the records are unclear on this point.

The division was back in Sofia by March 1943, except for the 25th Regiment which was in Thrace on occupation duty.

Upon the Bulgarian defection from the Axis, the First Infantry Division joined with Soviet forces in driving the Nazis out of Macedonia and later, Yugoslavia.