Q2. Why was “Congress of Vienna” considered a success?
The Congress of Vienna created enough powers of similar strength and influence that none of them could go too far without being overwhelmed by a coalition of the others. It mediated numerous tensions and conflicting interest through peaceful negotiations. The Congress also stopped potentially explosive issues from getting out of hand: the Poland issue could have led to war or further hostility, but it was handled with extreme care by a group of very capable diplomats. The Congress brilliantly established long-term stability in Europe.
Furthermore, the Congress created so little hard feeling and dispute that the whole of Europe did not all go to war at once for a century. Not until World War I broke out in 1914 did a massive, Europe-wide conflict occur. In that sense, the Congress of Vienna was a triumph of diplomacy.
The Congress of Vienna also decided to outlaw the Atlantic slave trade. All of the major powers agreed to this, but only Britain actually did anything to stop the trade, setting up an anti-slaving naval squadron.