Can't leave a hippo hanging.
We do not know what our chances of survival are, so we fight as if they were zero. We do not know what we are facing, so we fight as if it was the dark gods themselves. No one will remember us now and we may never be buried beneath Orbis, so we will build our own memorial here. The Imperium might lose us and Orbis might never know we existed, but the enemy - the enemy will know. The enemy will remember. We will hurt it so badly that it will never forget us until the stars burn out and the Emperor vanquishes it at the end of time. When ODN is dying, its last thought will be of us. That is our memorial - carved into the heart of ODN. We cannot lose, Astra Militarum. We have already won.
Tl;dr: we are marching to war in defense of The Golden Horde