In the second half of the match, the team was forced onto the defensive.
There is no chance of winning if we stay on the defensive.
Facing objections, he was put on the defensive.
In the second half, the team turned defensive.
Under the barrage of tough questions, the minister was forced onto the defensive.
防戦 refers to the act of fighting defensively, while 守勢 refers more broadly to the overall defensive stance or position one is in.
攻勢 means being on the offensive, the direct opposite of 守勢, which means being on the defensive.
守備 refers to the concrete act or duty of defending (e.g., fielding in baseball), while 守勢 refers to the abstract posture of being on the defensive in a struggle.
Compound of 守 (しゅ, 'defend') and 勢 (せい, 'force' or 'momentum'), literally 'defensive momentum'. The reading is on'yomi.