1) Most important changes in the Middle East between 13th & 15th centuries?

2) What caused the Chinese decision to abandon major expeditions?

Important changes in the Middle East between the 13th and 15th centuries include the 13 Century end of the First Crusade and the capture/founding of Jerusalem. After that came the rise of the Mongol/Turkish/Ottoman Empire during the 14th Century. The Ottoman transcontinental Empire controlled much of North Africa, Western Asia, and Southeast Europe. This led to the creation of important transcontinental trade routes and a boom of economic trade between the continents.

Pushing the Mongols north and fighting them, and the rebuilding of Beijing caused the Chinese decision to abandon major expeditions.