As one of China's earliest ministers to London, Paris and Saint Petersburg, he played an important role in the diplomacy that preceded and accompanied the Sino-French War.

The resulting Treaty of Saint Petersburg (February 1881), which reversed most of the Russian gains of 1879, was generally considered a diplomatic triumph for China.

[2][3] Zeng's duties as minister to Paris were dominated by the confrontation between France and China over Tonkin that eventually culminated in the Sino-French War.

[4] In August 1883, during a series of discussions in Paris with the French foreign minister Paul-Armand Challemel-Lacour, Zeng used the good offices of the American chargé d'affaires E. J. Brulatour to convey Chinese proposals to the French, in an attempt to give the impression that the United States was more closely associated with China's diplomatic position on Tonkin than it really was.

[5] In January 1884, in the wake of Admiral Amédée Courbet's capture of Sơn Tây (16 December 1883), Zeng wrote a provocative article that made wounding references to the Franco-Prussian War ('The bravery of the French soldiers has been so widely praised that one might almost think that they had captured Metz or Strasbourg rather than Sơn Tây').