The Weight of Command by Michael Mammay

Lieutenant Kiera Markov is a scout platoon leader for a peacekeeping force on the remote planet of Tanara, where little has happened for decades, and the only mission is to keep the lithium flowing up the space elevator to feed the galaxy's incessant demand. But when an unprecedented attack kills the entirety of the brigade's leadership, the untested lieutenant suddenly finds herself in command.
Isolated and alone, Markov must contend with rival politicians on both sides of the border, all of whom have suspect motives and reason to take advantage of an untested leader, while an unseen enemy seeks to drive the two sides toward a war that Markov has a mission to prevent. It's enough to test even a seasoned leader.
Markov isn't that.
With challenges from all sides, and even from her own troops, Markov will have to learn quickly and establish her authority. Because what hangs in the balance is not only the future of the peacekeeping force, but of the planet itself.