War with Wainriders - "The third evil was the invasion of the Wainriders, which sapped the waning strength of Gondor in wars that lasted almost a hundred years.  The Wainriders were a people, or a confederacy of many peoples, that came from the East (Easterlings); but they were stronger and better armed than any that had appeared before.  They journeyed in great wains, and their chieftains fought in chariots.  Stirred up, they made a sudden assault upon Gondor, and King Narmacil II was slain in battle with them beyond Anduin in 1856.  The people of eastern and southern Rhovanion enslaved; and the frontiers of Gondor were for the time withdrawn to the Anduin and the Emyn Muil.  [At this time it is thought that the Ringwraiths re-entered Morder.]" ~The Return of the King (Appendix A)