main force; main troops
Military term for the primary combat unit or the bulk of one's forces, as opposed to support or reserve units.
The enemy's main force is massing beyond the mountains.
We must hold out until the main troops arrive.
主力 is a broader term meaning 'main strength' or 'mainstay', used in military, sports, and business contexts. 主力部隊 specifically refers to the main military unit or troops.
部隊 means 'unit' or 'corps' in a military context. 主力部隊 combines this with 主力 to specify the main combat unit.
Compound of 主力 (main strength) and 部隊 (military unit).