I suggest it could refer to this JFK speech, which to me seems relevant particularly now we know the world is being swallowed whole by an unelected global communist UN/WEF government:

"The Communist balance of power is still strong. The balance of power is still on the side of freedom. We are still the keystone in the arch of freedom, and I think we will continue to do as we have done in our past, our duty, and the people of Texas will be in the lead." * 22nd Nov 1963 https://www.americanrhetoric.com/speeches/jfkfortworthcocommerce.htm

Other people's thoughts: https://theyig.ning.com/q/what-is-the-keystone