In what way did the anchluss present a challenge to the united states and its allies

It was a direct result of the failure of the American politicians to back Woodrow Wilson sufficiently to ensure a fair and equal application of the Versailles treaty. Under that treaty, self determination for national groups was supposed to have been available to the Germans in Europe, just as much as the other nationalities. Vienna and Western Austria should have been incorporated in Germany from the start, and the grievance which resulted from denying this right gave the nazi party a huge boost.