March or Die - The Fifth Foreign Legion, #1

Stranded in a Galactic Rebellion As the Terran Commonwealth expanded the borders of the interstellar territories it controlled, a waning alien race was forced to yield much of their once glorious star empire. But they had never been fully conquered, and they would never cease trying to undermine the Commonwealth's power. When Commonweath diplomats and the command officers of the Terran Fifth Foreign Legion gathered in the alien capital city to meet with a local ruler, no one suspected the trap they were walking into.
As assassins attacked the diplomatic party, native forces struck the fortress of the Fifth Foreign Legion, which was still under construction and not yet secure. The ranking surviving officer from the fort has to lead his troops on an impossible march through the heart of enemy territory, fighting every step of the way in a last-ditch effort to reach the main base . or die in the attempt.