According to script, the Borg failed to assimilate any 8472. Therefore, they did no harm to 8472. In the real world, nations will tolerate open violations of their sovereignty and even the deaths of their own nationals yet not go to war. This is true not just of weak countries like Pakistan but even powerful ones like the US (the Liberty, the Stark.)
The notion that 8472 was defending itself by invading normal space and slaughtering thousands (or was it millions? billions?) in response to no loss of life or property
is equivalent to saying that trespass can be justly treated as a capital crime! Their seems to be some projection involved here.