In 立下了汗马功劳, how should I understand 立下了 ("set up") 汗马功劳 ("war exploits")? In many example sentences for the chengyu

汗马功劳
hànmǎgōngláo
war exploits; (fig.) heroic contribution[src]

we see 立下了汗马功劳, e.g.:

他参加过许多战役, 立下了汗马功劳, 但他仍然很谦虚。

I don't understand this because 立下 = &quot;to set up; to establish&quot;[src].  It doesn't make sense to me how someone can &quot;set up war exploits&quot;, etc.  I'm sure I'm misunderstanding something here.
Question: How should I understand 立下了 (&quot;set up&quot;) 汗马功劳 (&quot;war exploits&quot;)?
立下 can be understood in a more smooth manner. It can represent set up as you said, when trying to express set up a contract or so. Examples include 立下誓言。
But in general it can also generalize to mean create, or make something happen. 立下誓言=make the 誓言 happen，立下汗马功劳=make the 汗马功劳 happen，etc。
Also, as Ho pointed out in the answer, 立功 is a common collocation, as earn credits for one's contribution. However I disagree with his understanding on properly usage of the 汗马功劳. Your example is completely correct in modern Chinese, and holds even in its original meaning (which specifies on military contributions).