This is basically the motives behind the Wampanoag decision to finally let the English setting in eastern Massachusetts. English fishermen, traders, and slavers had been sailing along the coast of New England for years before the Plymouth colony was established. That's how Tisquantum (Squanto) and Samoset learned to speak English, after all. Before 1620, however, the various New England nations consistently refused to permit the English to establish any permanent base of operations in their territory.

Between 1616 and 1618, an epidemic spread along the eastern seaboard - from New England down to the Virginia. We're still not sure what disease this was, though we know it wasn't any of the major introduced diseases. The current common hypothesis is that it was an outbreak of leptospirosis brought to the area by rats onboard the various European ships. You'll still see some older sources saying it was smallpox, but the first smallpox in the region appears to have happened in the 1630s, and the indigenous descriptions of the earlier disease are quite different from smallpox.

Southwestern Massachusetts was especially hard hit. Tisquantum's home town of Patuxet was completely abandoned due to this epidemic before he escaped captivity and returned home. When you here stories about 90% of Native Americans being wiped out by diseases before Europeans showed up, this is what they're actually referring to - a very small region in the years immediately before Plymouth. Further west, the impact of the epidemic drops off. The Narragansets in Rhode Island lost about 50% of their population, while the Pequots in Connecticut lost about 10%. By the time you get as far south as Virginia, we're dealing with only a handful of individual cases.

So the Wampanoag, finding themselves with an extreme numerical disadvantage compared to their neighbors, were looking for something that could level the playing field if the Narragansetts or Pequots decided to make a play for their territory. When the Plymouth colonists arrive and set themselves up in remains of Patuxet, the Wampanoag figured they had the solution to their problem. Massasoit Ousamequin negotiated trade and mutual defense agreements with the English in exchange for allowing them to remain in Patuxet.

Incidentally, it was the defense agreements they had that sparked the "first Thanksgiving" stories about the Pilgrims and Wampanoag feasting together. In this post, I went over how that all went down, at least according what few contemporary sources we have on the topic.