Learn Spanish (Latin America): No lo hagamos. A lo mejor no es pacífico. - Let's not do it. Maybe it isn't peaceful.

Because this is a negative command. For affirmative "let's" commands, pronouns attach to the end of the verb (like hagámoslo). But for negative commands, the pronoun must go between no and the verb, giving us no lo hagamos.

How do we get the verb form hagamos?

Hagamos is the nosotros (we) present subjunctive form of hacer (to do/make), which we use for "let's" commands. You form it by taking the present tense "yo" form (hago), dropping the "o", and adding the opposite ending for an -er verb (-amos).

Doesn't a lo mejor literally mean "to the best"?

Literally, yes, it translates word-for-word as "to the best" or "at the best." However, in everyday Spanish, a lo mejor is a very common fixed phrase that simply means "maybe" or "perhaps."

If a lo mejor expresses doubt ("maybe"), why does it use the indicative es instead of the subjunctive sea?