In what way is the formal amendment process an example of federalism at work

According to the National Archives "The Constitution provides that an amendment may be proposed either by the Congress with a two-thirds majority vote in both the House of Representatives and the Senate or by a constitutional convention called for by two-thirds of the State legislatures."

So, this provides for a Federal and a State option for passing amendments.

they wanted the u.s. to fight on the side of the central powers.