What was the purpose for the naval expeditions of the ming dynasty?

 Between 1405 and 1433, the Ming government sponsored a series of seven naval expeditions. Emperor Yongle designed them to establish a Chinese presence, impose imperial control over trade, and impress foreign peoples in the Indian Ocean basin. He also might have wanted to extend the tributary system. 

Zheng He was placed as the admiral in control of the huge fleet and armed forces that undertook these expeditions. Zheng He's first voyage consisted of a fleet of around 300 treasure ships holding almost 28,000 crewmen. 

Zheng He's fleets visited Arabia, East Africa, India, Indonesia and Thailand (at the time called Siam), dispensing and receiving goods along the way. Zheng He presented gifts of gold, silver, porcelain and silk; in return, China received such novelties as ostriches, zebras, camels, ivory and giraffes. 

Zheng He generally sought to attain his goals through diplomacy, and his large army awed most would-be enemies into submission. But a contemporary reported that Zheng He "walked like a tiger" and did not shrink from violence when he considered it necessary to impress foreign peoples with China's military might. He ruthlessly suppressed pirates who had long plagued Chinese and southeast Asian waters. He also waged a land war against the Kingdom of Kotte in Ceylon, and he made displays of military force when local officials threatened his fleet in Arabia and East Africa. From his fourth voyage, he brought envoys from thirty states who traveled to China and paid their respects at the Ming court. 

In 1424, the Yongle Emperor died. His successor, the Hongxi Emperor (reigned 1424–1425), decided to stop the voyages during his short reign. Zheng He made one more voyage under the Xuande Emperor (reigned 1426–1435), but after that the voyages of the Chinese treasure ship fleets were ended. Z 

Zheng He, on his seven voyages, successfully relocated large numbers of Chinese Muslims to the nascent Malacca, which became a large international trade center.

the united states broke it's neutrality was the constant german sinking of trading vessels resulting in the loss of lives, one example would be the lusitania.