The primary issue with a one party system is the suppression of individual rights and no democratic rights.
In a one-party system, like the one in Communist China, the people have no voice in how the country is run. One political party makes all the decision without any opposition in the parliament.
From the perspective of human rights, this will lead to dictatorship and a curb on freedom of speech.
Are you Australian? In that case it would rather ironically be the Aboriginals and their children. If not, (and I'm not 100 percent sure about this one) but this was during the time period of the African American rights Reform period in the United States.
Well the Tang made China's government stronger. They went back to using the civil service examinations for choosing government officials. Government officials could no longer use family connections to get them jobs. They also gave land to farmers and brought back order to the countryside. The effects helped China rebuild itself, offering better standers of living to its people.
That the Schlieffen plan failed, The French were unable to defeat the Germans completely at the Battle of the Marne.