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.