They were so pissed off by what Janeway did in Endgame, that they still had the force of arms left (7461 vessels) to launch a full scale invasion of the Alpha Quadrant and kill, not assimilate, killed 40 percent of the Federation (63 Billion with a B people.)
http://memory-beta.wikia.com/wiki/Borg_Invasion_of_2381
But unfortunately, They used up so many resources in this invasion that they lost everything and died out.
"Before you embark on a journey of revenge, dig two graves."
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