Final answer:
Based on the Tenth Amendment and Article I, Section 10 of the Constitution, state governments would not have the constitutional authority to create their own armies or start their own wars if Congress declared war on another nation.
Step-by-step explanation:
Based on the Tenth Amendment and Article I, Section 10 of the Constitution, if Congress declares war on another nation, state governments would not have the constitutional authority to create their own armies or start their own wars. The Tenth Amendment reserves powers to the states that are not delegated to the federal government, but the power to declare war is specifically granted to Congress in Article I, Section 8 of the Constitution. Therefore, any state government policies related to starting wars would be unconstitutional.