Here is the relevant rule, from Gygax's DMG p 70:
Breaking Off From Melee:
At such times as any creature decides, it can break off the engagement and flee the melee. To do, however, allows the opponent a free attack or attack routine. This attack is calculated as if it were a rear attack upon a stunned opponent.
And how is this sort of play - ie in which PCs hire or summon "muscle", and then sacrifice it when expedience demands -
better than play in which the protagonists fight their own fights?
Answer: it's not. My impression is that the number of contemporary RPGers interested in this particular approach is very much a minority.