Hola Kevin

This is a very interesting question. This sentence is a "se impersonal" sentence (not following the construction of a reflexive se sentence). We keep the verb in the singular form even when the object is plural, in this case: "los enemigos". 

We do this when the object is "personal" (as opposed to a thing) and requires the preposition "a". 

Here is an article that talks about this from the R.A.E that explains this grammatical point. (The answer is on the last two bullet points)

I hope it helps.

Saludos cordiales

Inma