Just because something didn't last long doesn't mean it was weak, or vice versa.
If, for example, the Union let the Confederates go, they might have survived for longer. But then, a confederated country is not a very good one, so I think it would have either consolidated or be an ineffectual state, like the H.R.E.
On the other hand, the Roman Empire was weak towards its end after Constantinople was sacked by the Venetians, yet it survived another two hundred years, until they finally fell to the Ottomans in 1453.
