Lief Erikson brought men and women, along with some wheat and horses, on his venture to Vinland in ~1000 CE, thanks to his family being more highly respected before his exile.
As a result, his settlement thrives long enough to be a port-of-call in the new world. As heretical pagans invade scandinavia, the Norse begin to migrate west in small number.
At first, relations with the native "Scraelings" are warm, and no small number of Norse men take native brides, and vice versa (Norse women had some measure of freedom when selecting husbands); some Norse smiths set up forges. As they learn each other's languages, the Norse tell of christian savages back in Europe. After a few years, a great sickness sweeps the natives, leaving the Norse colonists nearly untouched- a charismatic leader of a local tribe determines that the Norse colonists are to blame, and things turn sour.
There are a number of skirmishes before the Norse colony is ultimately wiped out, except for the few who have married into local clans. The conflict drives migrants south and west across the continent, bringing the plague, and the genes to resist it, with them.
Five hundred years later...
The first Christian explorers of the New World arrive on the shores of America, Set down the flag of Spain, and are immediately struck down by a flying, spinning, <<*Steel Tomahawk*>>.
The population of native Americans, though lower than in OTL at the time, is now mostly immune to western disease, already has the horse and steel, and consider white-skinned boat people to be demons bent on spreading a false Religion. There are still local divisions between clans and kingdoms, even wars, but the growing threat of European incursions is a powerful unifier....