Refers to a body of soldiers or an army, often with a sense of scale or strength. Can also imply the number of troops.
He gathered his army around the castle.
The enemy forces approached the castle.
He led a large army into battle.
The general led a large army into battle.
The enemy's army surged toward the border.
An army of ten thousand surrounded the castle.
兵力 stresses military strength or troop numbers as an abstract resource, whereas 軍勢 refers to the actual body of soldiers gathered together.
兵隊 usually means soldiers or troops in a plain, everyday sense, while 軍勢 conveys a large assembled force, often in historical or dramatic contexts.
軍政 (ぐんせい, military government) sounds similar to 軍勢 (ぐんぜい, a body of troops or military forces), but the two are unrelated in meaning.
Compound of 軍 (army, military) and 勢 (force, power, energy). The combination conveys the idea of a military force or the strength of an army.