When hostilities broke out he joined up.

战争爆发后，他参了军。

《柯林斯英汉双解大词典》

Hostilities broke out last month when bulldozers, accompanied by armour-clad police and plain-clothes thugs, entered Rechnik at 4am.

双方敌对情绪于上月爆发，开始时推土机、顶盔贯甲的警察及衣着平常的恶徒在凌晨四时浩浩荡荡进驻里芝柯。

youdao

Russia's RTS Index of leading stocks has fallen nearly 30 percent since hostilities broke out in Georgia on August 7 and 50 percent since June 1.

自从俄罗斯和格鲁吉亚在八月7日爆发武装冲突以来，以俄罗斯主要上市公司组成的俄罗斯交易系统指数暴跌了将近30%。 如果从六月1日算起，这个指数已经下滑了50%。

youdao

Hostilities broke out in February, when Mr. Slim's entire stable of companies pulled their advertising from Televisa, arguing rates were too high.

WSJ: Rare Headwinds in Mexico Buffet Slim

Roger Evans, a Catholic deacon from Chislehurst, England, was on a tour of the Holy Land with nine others from his parish when the hostilities in Gaza broke out in November.

CNN: Gaza conflict deters Bethlehem Christmas pilgrims