"power must never be trusted without a check" what does this refer to

From John Adams, with Postscript by Abigail Adams

Explanation:

The poles, due to fact that till 18th centruy and since 15th poalnd was european great power and for some time even   the most powefrull on teh contintent with much economic wealth and rich culture (also multiethnic poalnd in eraly modern era  was very similar to the usa in late 18th cnetury) after the lost of indpendence in which russia was major contributor that part of poalnd which was gained by empire was highly russianized and culture war against polsih culture was held