Quote Originally Posted by heartfelty View Post
The Congress System has been roundly condemned by many hisorians as a tool of reaction.

Case is Past Imperfective since it began in the past and still continued to the past ..
Do you mean "tense"?
Quote Originally Posted by heartfelty View Post
Congress System is nominative.
That's correct
Quote Originally Posted by heartfelty View Post
'by many historians' is dative since it answers by whom or by what.
"by" most of the time means instrumental case as in here
Quote Originally Posted by heartfelty View Post
'as a tool of reaction' is instrumental since it answers 'how'
this is incorrect and I'm not sure why instrumental answers "how".
And also "a tool of reaction" is "what" per my understanding
It is nominative here

AFAIK - The Congress System is this specific thing - Concert of Europe - Wikipedia, the free encyclopedia
And in Russian it is translated as Система Европейского концерта or Венская система международных отношений

Система Европейского концерта была резко осуждена (или резко осуждаема) многими историками как инструмент реакции

BTW I have no idea what "a tool of reaction" could possibly mean