Argthtjtr

I wonder about the use of the word "Union" in the name "State of the Union", which is the US President's annual address to Congress. The phrase appears in the Constitution:

He [the President] shall from time to time give to the Congress
Information on the State of the Union,

However, the phrase apparently fell out of favor for many years; according to the Senate's FAQ

The message was generally known as “the President’s Annual Message to Congress”
until well into the 20th century. Although some historians suggest that the phrase “State
of the Union” emerged only after World War II, President Franklin Roosevelt’s 1934
message is identified in his papers as his “Annual Message to Congress on the State of
the Union.” According to the Office of the Clerk of the House of Representatives, it was
known informally as the State of the Union Address from 1942 to 1946, and has been
known “generally” by the same name since 1947.

The choice of the word "union" seems to emphasize that it's about how the states are getting together; this would seem to be more appropriate during the period when the country was forming, to refer to the process of uniting. Why isn't it "State of the Nation" or "State of the Country"? The states also have similar traditions where the governor gives an annual address, and they generally call it "State of the State" (or "State of the Commonwealth" in a few states that identify themselves as such).

Oxford Living Dictionary does include this definition of "Union":

4.1 The United States, especially from its founding by the original thirteen states in 1787–90 to the secession of the Confederate states in 1860–1.

But in my experience, this usage is extremely rare; I can't actually think of any usage other than this specific phrase. Was it a more common way to refer to the US a century ago, and it has now become entrenched in the name of the speech, while falling into disuse in other contexts?

I assume it fell out of favor to refer to the entire nation because during the Civil War it was used to refer to the northern states, so it seems like it would have had a bad connotation to southerners. Could it be that by the 1930's there were few people still alive who lived through that war, so it was acceptable to resurrect it in a more positive light? Yet it's been about as long now since World War II, and few would think of trying to use Nazi, Fascist, or Axis positively.