Weakening the US politically, and ending the USD's dominance in global finance.

The first is already happening, as is the second. The domination they have engineered and fuelled with debt, will be its downfall.

It's already the greatest nation on earth at the time you assumed office.

Militarily and by influence globally, but not sure if internally. So many issues from a highly-unbalanced economy with more and more people falling into poverty. Eventually, it has to break, like all such structures in the past.