Zheng He (Chinese: Zheng He ; 1371-1433 or 1435) was a Chinese mariner, explorer, diplomat, fleet admiral, and court eunuch during China's early Ming dynasty. He was originally born as Ma He in a Muslim family, later adopted the conferred surname Zheng from Emperor Yongle. Zheng commanded expeditionary voyages to Southeast Asia, South Asia, Western Asia, and East Africa from 1405 to 1433. His larger ships stretched 120 meters or more in length.

In the decades after the last voyage, Imperial officials minimized the importance of Zheng He and his expeditions throughout the many regnal and dynastic histories they compiled. The information in the Yongle and Xuande Emperors' official annals was incomplete and even erroneous; other official publications omitted them completely. Although some have seen this as a conspiracy seeking to eliminate memories of the voyages, it is likely that the records were dispersed throughout several departments and the expeditions - unauthorized by (and in fact, counter to) the injunctions of the dynastic founder - presented a kind of embarrassment to the dynasty.  State-sponsored Ming naval efforts declined dramatically after Zheng's voyages. Starting in the early 15th century, China experienced increasing pressure from the surviving Yuan Mongols from the north. The relocation of the capital north to Beijing exacerbated this threat dramatically. At considerable expense, China launched annual military expeditions from Beijing to weaken the Mongolians. The expenditures necessary for these land campaigns directly competed with the funds necessary to continue naval expeditions. Further, in 1449, Mongolian cavalry ambushed a land expedition personally led by the Zhengtong Emperor at Tumu Fortress, less than a day's march from the walls of the capital. The Mongolians wiped out the Chinese army and captured the emperor. This battle had two salient effects. First, it demonstrated the clear threat posed by the northern nomads. Second, the Mongols caused a political crisis in China when they released the emperor after his half-brother had already ascended and declared the new Jingtai era. Not until 1457 and the restoration of the former emperor did political stability return. Upon his return to power, China abandoned the strategy of annual land expeditions and instead embarked upon a massive and expensive expansion of the Great Wall of China. In this environment, funding for naval expeditions simply did not happen.  However, missions from Southeast Asia continued to arrive for decades. Depending on local conditions, they could reach such frequency that the court found it necessary to restrict them: the History of Ming records imperial edicts forbidding Java, Champa, and Siam from sending their envoys more often than once every three years.Answer this question using a quote from the following article:

why were they fighting the mongolians
Starting in the early 15th century, China experienced increasing pressure from the surviving Yuan Mongols from the north.