What is the fundamental disagreement between sunni and shia muslims?

They maintain that Ali, Muhammad's cousin and son-in-law, was the rightful heir to the leadership of the Islam religion after Muhammad's death. Another contentious religious difference between Shiite and Sunni Muslims concerns the Mahdi, which is Arabic for "guided one." (livescience.com)

Both sides agreed that Allah is the one true God and that Muhammad was his messenger, but one group (which eventually became the Shiites) felt Muhammad's successor should be someone in his bloodline, while the other (which became the Sunnis) felt a pious individual who would follow the Prophet's customs was acceptable.

Explanation:

the qing dynasty placed strict limits on foreign merchants' ability to trade in china.