After the war broke out, the country quickly re-armed its troops with modern tanks.

re-armed 是动词 re-arm 的过去式形式。re-arm 由前缀 re-（表示"再次、重新"）和动词 arm（武装、装备）组成，字面意思是"重新武装"。re-armed 表示"重新装备了武器"的意思。