Put it this way:
Fight, for your lives depend upon it.
The survivors shall be heroes among the noble realms - gaining the fame, honor, glory, and prestige to elevate them unto new realms of power and influence.
The dead shall reign as honored martyrs throughout time immemorial, their names inscribed evermore upon the tapestry of fate's delusion.
Mere knights, after a number of victories, will hold the power (prestige/etc.) to become generals, judges, ambassadors, and rulers.
New commands shall arise. Stagnation shall fade. Borders will shift. The world will never be the same - and we shall be all the better for it.
The remnants of this invasion shall pave the way to future empires. Rogue lands can be reconquered and become the site of new realms, divergent colonies, and present the new face of continental politics.
Weakened cities and realms will present fruitful targets for numerous wars, further adding to the chaos and subsequent re-establishment of order that shall inevitably follow.
Consider it a sign from the Gods that an Age of Reckoning and Metamorphosis is upon us.
