In the 20th century, many historians also agreed with the achievements of the conference, such as leniency towards defeated countries, establishment of a conventional framework for coordination and cooperation in Europe, abolition of the slave trade and opening of international rivers, all of which contributed to the reconstruction of peace and the progress of European society. They believe that the decision of this congress has made it nearly a hundred years (from 18 18 to 19 14). Among these historians is Henry Kissinger, whose doctoral thesis is about the Vienna Conference. A peace treaty to end the Crimean war. 1856 From February 25th, Britain, France, Sardinia, Turkey, Russia and Austria, the initiator of the Crimean War, held a meeting. After discussing the Black Sea Strait issue, Prussia, one of the signatories of 184 1 London Strait Convention, was invited to participate. On March 30th, the Paris Peace Treaty was signed. According to the peace treaty, European powers formally recognized Turkey as a "European Alliant" and guaranteed the "independence and integrity" of the Ottoman Empire. Russia returned the mouth of the Danube River and South Bissalabia to Moldavia. Give up the right to protect the Orthodox Christians in the Ottoman Empire (including Moldavia and Wallachia), and the powers will jointly protect their interests. Moldavia and Wallachia are still under the jurisdiction of Turkish suzerainty and Serbia is autonomous, but the Turkish Sultan still retains the supreme power over Serbia; Russia returned Kars to Turkey and recovered Sevastopol and other cities in Crimea occupied by the British and French allied forces. The Black Sea is neutral. The Black Sea Strait (Bosporus Strait and Da Daniil Strait) is closed to warships of all countries. Neither Russia nor Turkey can keep more than six 800-ton ships and four 200-ton ships in the Black Sea, nor can they build naval arsenals and naval fortresses along the coast of the Black Sea. Trade and navigation on the Danube River are completely free and open to all countries. The "European Danube Commission" composed of Britain, Russia, France, Austria, Hungary, Sardinia and Turkey ensures the implementation of this regulation.