Eru didn't send them, just as he didn't send the Valar and Maiar to wage war against Morgoth that caused the cataclysm you mentioned. Eru left Arda to ruled and watched over by the Ainur (Valar and Maiar) well dwelling himself outside of all time and creation in the void. the Valar decided to send the Istari, forbade them for fighting Sauron power against power in order to teach the free people and guide them to victory by their own hands...so no other cataclysm of that nature would occur. their power and their memories of the Undying Land both were limited when they were bound to mortal incarnate flesh. Eru's revival of Gandalf was one of few instances where he directly intervenes. because Eru knew without Gandalf the free people's would fall, and his children would be doomed. so, he made him the white, unlocked a greater measure of his natural power and returned him with adjusted rules. allowing Olorin to in certain situations, if things were dire enough to directly confront the forces of the Dark Lord with his power. which he does on several occasions after his return. ... but, long story short, and to get back to my point. Eru didn't send the five Istari in the first place, nor did he send the Ainur to confront and wage war against Morgoth. both acts were decisions made by Manwe, Leader of the Ainur, King of the Valar, and Morgoth's brother.