1919年3月間的天津租界美日衝突案顯然是一個相當值得深入探究的事件，適足以用來檢討一戰後美、日在華互動的新格局，同時也可以檢視在中日山東問題之爭與反日運動的刺激下，中、美、日三角間的複雜關係。本文利用美、日雙方第一手的檔案資料，深入追查事件背後所體現的重要意涵。特別是天津美日衝突事件發生的時間相當敏感，正值山東問題爭議已在巴黎和會引起中、日代表團的言語交鋒與立場對立，美國代表團在威爾遜總統的率領下，明顯較為同情中國。美國駐華官員以及民間社群，也是不太隱藏其對日本野心的猜忌與厭惡。日本外務省則開始在日本本地以及中國發動一系列的反美宣傳，以制衡美國，並為巴黎和會上的外交行動張勢。而中國的反日運動此時則仍在醞釀之中，對日本的不滿日益高漲，各地也零星出現抵制活動，但尚未爆發大規模、有組織的反日運動。換言之，此案恰巧提供一個很好的分析點，可以探究一戰後美方的對日態度，並藉此略窺在五四政治運動發生之前，美國駐華官員對於日本的觀察，以及如何看待一戰後的美、日在華互動。

American-Japanese conflict in the Tientsin Concession in March 1919 is an important event. Through deep investigation of this event, we can re-examine American-Japanese interaction in China after World War I, as well as the triple-complex relationship between the U.S., Japan and China in the contexts of Shantung Issue and the anti-Japanese movement in China. This paper utilizes the original archives of U.S. and Japan to trace historical implications behind the event. It shows that the event was very crucial as the controversy over the Shantung problem was initiated in the Paris Peace Conference. The dispute between the Chinese delegation and Japanese delegation drew the world’s attention, and Woodrow Wilson, then the President of the U.S., showed his sympathy with the Chinese. American diplomats, military officers and civilians in China also expressed their doubt about Japan’s ambition. On the other side, Japanese Government seemed to launch anti-American propaganda in China and Japan to counteract the U.S. and tried to direct public opinion as a method to express their support of the diplomatic negotiation. Meanwhile, despite that there was no large mass movement against Japan, the Chinese hatred of Japan gradually came into place as a result of the 21 Demands in 1915. To sum up, the Tientsin incident offered a good opportunity to analyze American officials’ observations of Japan in China, and to evaluate how the US government weighed the American-Japan relationship in China after WWI.