After years of rivalry, he finally let his guard down and offered a truce.

let 是动词 let 的原形，意思是"允许、让"。 his 是代词 he 的所有格形式，意思是"他的"。 guard 是名词，意思是"守卫、防护"。 down 是副词，意思是"向下"。 let one's guard down 是一个固定短语，字面意思是"放下某人的防护"，引申为"放松警惕、卸下防备"。