The MM have a strong strategic position and much more advanced technology and weaponry than the Grounders. However they have never wiped them out, mainly because they needed their blood but also because they couldn't stay outside so they weren't competing for their land.

By giving the MM the Arkers, they give them the means to never have to take Grounder blood again (sounds good right...) while also giving them the ability to come out onto the ground like they have always wanted. So now they have no direct need for the Grounders and are going to be competing for their land, it seems logical that the MM will begin a process of killing or forcibly relocating the Grounders. Their actions have shown that they are impatient and certainly not above killing innocents lets alone the savages they consider the Grounders to be. The Grounders can expect a missile onto their capital very soon.

As a commander of their military Lexa would have to have some concept of acceptable losses. She already lost way more people in the missile strike than the amount of hostages MW had, she considered that to be an acceptable loss for the battle to come. Yet in the moment of the battle, where she has the biggest advantage her people have ever had over the MM she decides that every life is sacred and it isn't worth the blood shed. I must be watching a different show to some of you if you think that makes sense.

I also think talk of the Grounders and Arkers charging into machine gun fire like it's WWI is a pretty poor excuse. The Arkers should have at least as good weaponry as the MM and coming from the Ark should have a decent idea of combat in a confined space, while the MM were shown not only to be mindless warriors but to be basically forest ninjas. So they should not be so incompetent that they can't overcome a far smaller force, even if it takes them some time to do so.

I'll just have to hope this is all a ploy by the writers and Lexa has some other plan we are not aware of.