The Emperor Constantine negotiated a liberal immigration treaty with the Germanics that resulted in a massive influx that split the empire between Rome and Constantinople. Rome fell soon after.

It may have been more of a concession than a negotiation since they would have probably invaded if they had been refused but still...

Mary Ann is our resident history expert but that is the gist of it. The intention was to absorb the Germanics but because they were allowed to come armed and there were so many, the area they occupied was soon a defacto Germanic sovreignty.

In the end it is said that the Roman Empire fell because the Romans disappeared, overwhelmed more by culture than military force.