Is "but rather" grammatically correct? [duplicate]

Is the use of "but rather" correct here?

The United States' decision to drop atomic bombs on Hiroshima and Nagasaki was not a diplomatic measure to intimidate the Soviet Union, but rather a military measure designed to force Japan’s unconditional surrender.

Yes it is. The but is necessary because the second part of the sentence is not a clause--it doesn't stand alone as a proposition. The rather itself approaches redundancy, but it does intensify contrast. In order to lose the but, the sentence needs to do something like this:

...not a diplomatic measure to intimidate the Soviet Union; rather, it was a military measure...

It might help to imagine a clunky pair of commas before and after the rather. Note nevertheless how little is lost by dropping the rather altogether.