Unity with highly advanced people (of the time) = Win
Unity with backward people (of the time) = Big loss
It is the rule of Allah swa, that He rewards hard working and forward looking people, and punishes the lazies and backward people. .
Thus it is imperative on us individually and collectively to seek the hardworking, forward looking people REGARDLESS of their religion. And if we don't then REGARDLESS of religion, death and destruction will be our fate. That's the whole Islamic concept of justice.