putin is not entirely to be blamed for the decline, but the signs of covert manipulations is all there. The very election of trump, and his poor foreign policies, are all smoking guns.
American institutions have only themselves to blame for the decline. People like Putin are taking advantage of the weakness but American politicians could behave more reasonably anytime but instead they are consciously choosing to make it worse. It’s all home made by the “greatest country in the world”. No need to blame anybody else.