I usually have a very good memory for details, but for this story I remember quite little.
It contained humanity/earth and some (one?) alien species. At some point an alien fleet was completely wiped out by some freak solar flare. Humanity claimed responsibility and said it was their super secret superweapon.
They then used the threat of using this superweapon to keep the peace. The weapon was supposedly split into multiple pieces and hidden, but could at any moment be reassembled.
I would expect that this is some sci-fi book from before 1990s, but I'm not sure. I am no longer sure if this was a major plot point, or merely some political setting for the story to take place in.