Whose defeat at the makeshift defensive structure known as Fort Necessity began the Seven Years' War?

George Washington, is the right answer.

Explanation:

George Washington, the then lieutenant colonel in Virginia, began the construction of a makeshift Fort Necessity.The necessity for constructing a makeshift emerged as a result of the murder of Joseph Coulon de Jumonville, a French military officer, in the custody of Washington.The French soldiers under the command of Jumonville’s half-brother forced Washington to confess for the murder of Jumonville and surrender.