Hell, Blizzard treats the Horde as aggressors even in the previous faction war even though they confirmed in Chronicle v3 that Alliance declared that war.
The Horde isn't obligated to react in any particular way to Alliance aggression, let alone react with kid gloves on. If the Alliance doesn't want to get its teeth kicked in during Horde's reaction, the onus is on them to give the Horde nothing to react to. Disproportionate response is a perfectly valid doctrine.
On top of that, prior to this "regional scuffle" there was this little event where Alliance forces under the orders of their High King attacked Forsaken fleet with Warchief of the Horde on board...