What “invisible invaders” were the natives, in the newly expanded territories, unable to fight against

answer : foreign infectious deceases, like small pox

Explanation:

to quote some artical i found when looking uo a proper answer to this, "By far the most drastic upheavals were caused by invisible invaders, the foreign bacteria introduced from the Old World. Native populations cut off from Europe, Africa, and Asia for millennia were utterly without immunity to smallpox, measles, and other ailments the newcomers unwittingly brought across the seas. "