What were the crusades why are they known as ''successful failures

The aim of the crusades was to take over the so called Holy Lands. Much of the ambition was to just loot the place instead of saving it. Sometimes there was a success and the crusaders held it but it was never held for very long. They did hold Jerusalem for over 75 years at one time. But that is just a drop in the time line for the Mideast. Eventually, the Crusaders were kicked out. Last, we still feel the effects of this effort. Many of the Islamic nations still remember it as if it was yesterday and they are not fond of those memories. Hence a successful failure.