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They brought back knowledge that had been lost before the crusades
There were a total of nine major crusades that took place between the 11th and 13th centuries. In addition to the major crusades, there were also several smaller, less significant crusades and expeditions.
Soldiers in the Crusades were exposed to many products that were unfamiliar to them in Europe. They learned the Muslim world had been trading even further east and that the possibility of much economic gain could be enjoyed by increasing eastern trade.
It could be argued that the Roman Empire has never fallen because its influence still remains with us even today.
To answer this question in the most simplest form you must look at the aim of the crusades; although there were many secondary aims for the crusades the main reason from the first four crusades to embark was to capture Jerusalm, which had been help by the Muslims since 638AD. So taking this into respect the first crusade must have been the most successful because it was the only one out of the first four crusades to capture Jerusalm!
how could apartheid have been prevented.
nope
It is highly unlikely. Once the man on the street entered the fray, change was bound to happen.
The Mexican oil spill could have been prevented. All oil spills could have been prevented if it weren't for careless people! An automatic switch should have been installed on the inside of the pipe.
This is an unanswerable question. There is no way to count "deaths that could have been prevented," let alone deaths that could have been prevented by an indirect correlate.
No
Yes, it is very likely that the planes crashing into the Twin Towers and the Pentagon could have been prevented.
The fire at the Triangle Shirtwaist Factory could certainly have been prevented with the knowledge and procedures we use today. Its death toll could have been reduced if the doors had not been locked and the switchboard on the 9th floor had been in operation. Whether the fire could have been completely prevented with the technology and procedures of the day is an open question that can never be completely answered.
If communist North Vietnam did not try to conquer South Vietnam, the war would have been prevented.
maybe
Yes.
It couldn't. Cyclones are a natural weather phenomenon which cannot be prevented or controlled.