Our Children's Children: Clifford Simak

If there weren't so many of the world's new visitors, nobody would believe their outrageous claim. That they come from the future. But they arrive in their millions - a constant stream of people escaping into the past to flee an invasion of murderously savage aliens; people to whom the world now has a responsibility, for they are our children's children. The refugees' time tunnels, the one-way passages that conduct them to a temporary haven on earth, are supposedly secure and adequately guarded by contemporary weapons. But when the alien monsters breach their defences the world is in for an orgy of indiscriminate warfare and slaughter...