If you get detail history, their has been full Dawah to king Rustam before battle.
There went at least three delegation of Sahaba on different days and gave Dawah of Islam to king Rusatm. But he did not accepted.
May one ask, which war took place in this century which converted millions of people to Islam?
Which sword is forcing people in the West to accept Islam in such numbers?
Requesting for Dua.