What precedent did washington establish by acting decisively to stop the whiskey rebellion

Use of state militia to quell insurrection.

Explanation:

In acting quickly to supress an insurrection that threatened the unity of the United States by the use of state militia, President Washington set a precedent that would eventually become law with the Insurrection Act.

By this precedent, Presidents are empowered to raise state militias to quell rebellion if normal civilian judicial procedure fails and after they have ordered the people involved to disperse.

This precedent has been used by quite a number of presidents including Ulysses Grant, John Kennedy and Dwight Eisenhower as they attempted to enforce laws calling for equality.