Why was "of" used in "Of a war"? If possible, what word would you put before "of a war" to help me understand the meaning better?

The only thing both sides agree on is that the best case is decades of estrangement—and that the worst, of a war, is growing ever more likely.

Question

Why was "of" used in "Of a war"? If possible, what word would you put before "of a war" to help me understand the meaning better?

The only thing both sides agree on is that the best case is decades of estrangement—and that the worst, of a war, is growing ever more likely.

Answer

As Adam said, "of a war" modifies "case". The sentence would be clearer if you just dropped the unnecessary words:

"...and that the worst, war, is growing ever more likely."

Also, I would replace "case" with "possible outcome".

Answer

There are two parallel trains of thought here to help you understand. There is the "case of decades of estrangement" and there is the "case of a war". I think the punctuation makes this sentence hard to read.