Category 1895 Treaty
Sub-Category Validity
Question Was the 1895 Treaty of Shimonoseki ever recognized as valid by the western powers?
Answer The 1895 Treaty of Shimonoseki specified the cession of "Formosa and the Pescadores" (aka “Taiwan”) to Japan. In regard to a formal recognition of the validity of this cession, most people look to the 1922 Washington Naval Treaty.

The Washington Naval Treaty, also known as the Five-Power Treaty, and the Nine-Power Treaty, was a treaty signed during 1922 among the major nations that had won World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Naval Conference, held in Washington, D.C., from November 1921 to February 1922.

In the 1922 Washington Naval Treaty, the United States and other signatories fully recognized Taiwan as being an insular area of Japan.

Article 19 of the treaty specified –

(3) The following insular territories and possessions of Japan in the Pacific Ocean, to wit: the Kurile Islands, the Bonin Islands, Amami-Oshima, the Loochoo Islands, Formosa and the Pescadores, and any insular territories or possessions in the Pacific Ocean which Japan may hereafter acquire.

Ratifications of that treaty were exchanged in Washington on August 17, 1923, and it was registered in the League of Nations Treaty Series on April 16, 1924.

Further References and Links
Taiwan's Legal Status: Taiwan's Legal Status: An Overview of the San Francisco Peace Treaty

Areas Conquered by U.S. Military Forces and therefore under USMG Jurisdiction -- with later "new disposition" by peace treaty