Which phrase does the protest document use to describe conscription?

The correct answer here would be that "conscription was despotism."

Despotism refers to the absolute rule of the a single entity who rules as it choose. That was usually a monarch but it can be any kind of government that obstructs the rights of its citizens. It is used to describe the abuse of power and the oppressing of people. Here conscription is despotism as the government is oppressing the people and their rights given to them by the 13th Amendment and it is abusing its power.