Visiting the aftermath of the Russo-Japanese war in 1905 Mitchell went for some time inspecting 'the area'. Stationed primarily for two years in the Philipines, he went on "an undercover reconnaissance mission of Japanese activities in the islands lying between Formosa and the Philippines". After visiting the battle fields of the 1905 war he studied Chinese, Russian and Japanese forces.
His conclusion were put forth into an official report to the War College Division of the General Staff in Washington.
His main points were summarised:
While Mitchell's predictions about Japan were significant primarily as landmarks in his personal development, some of his thoughts deserved consideration in Washington. American diplomatic policies in that area of the world, Mitchell stressed, were not isolationist. Mitchell warned that the constant display of American interest in Korea, Manchuria, and China ran counter to Japanese plans. Moreover, the United States had chosen to antagonize the most advanced of the Oriental nations—Japan—through a policy of discrimination against her nationals who had immigrated to America. American persistence in such policies, not to mention the "Open Door" policy in China, made it imperative that America make military preparations to back up her diplomacy.
Alfred F Hurley: "Billy Mitchell, Crusader for Air-Power", Indiana University Press: Bloomington, Indianapolis, 1964/1975, p 14.
(Source for this book cited as:
WM, "Report of Observations in Manchuria, Korea and Japan," NA, GS Report 7027-1, Jan. 2, 1912 (hereafter cited as "Report of Observations"), esp. 77-80; "Report to the Adjutant General, U.S. Army," NA, Report 7027-2, Mar. 22, 1912, 75; Outen J. Clinard, Japan's Influence on American Naval Power 1897-1917 (Berkeley, 1947), 2f.
That seems to make a commentary necessary, also in light of the way the linked question that originated this question is phrased:
It seems not to be the case that Mitchell saw war with Japan as inevitable!
It looks like Mitchell analysed both sides and concluded that there was a clear conflict of interests emerging, additionally fuelled by racial antagonisms, that would very probably lead to a confrontation if both sides kept their then current trajectory. So, the envisioned development was that not only was Japan a threat to the US, but the US a threat to Japan as well. Both displaying disrespect for each other and oogling on marbles that both claimed as theirs.
The University of Virginia had digitized that book, sold it to Google, and now it's lost to the public for viewing instead of being freely available on the net. This perversion must end in much less time than it took to destroy Carthage.