It was of course for the protection of the legation, but other main objectives included expansion of Japanese interests in China, establishing Japanese advantage at the Korean Peninsula by defeating the Qing dynasty, and restrainment of Russia which was sending a great army just like Japan, and Japan also dispatched troops alongside the allied western powers to show its presence as the 'military policeman of the Far East' (in order to correct the unequal treaty).
公使館の保護は無論であるが、中国における日本の権益拡大や、清朝を叩くことで朝鮮半島における日本のアドバンテージを確立すること、日本についで大軍を送っていたロシアへの牽制、列強側に立って派兵することで「極東の憲兵」としての存在感誇示（＝不平等条約改正）などが主要目的であった。