Why did Americans find the Zimmermann note so threatening?

An act of war

Explanation:

The U.S. was not interested in war, however with the greatly increasing amount of America ships sunk in unrestricted submarine warfare, The U.S. was pushing towards war. The British intercepted the Zimmerman note that the Germans hoped the Mexicans received. This note was very threatening to the U.S. since it called for Mexico to invade America, including Texas and its borders. The U.S. most definitely didn't want a war on their own soil, against a neighbor.

to unite muslims and hindus in resistance against british colonialism.

a system of cooperative federalism emerged, as the powers of the federal government expanded.