Poor judgement by Starfleet in overall military strategy.
Early loss of key facilities near the Klingon boarder. (Think of that like the Japanese attack on Pearl Harbor. The Japanese caused a lot of damage, but failed to destroy the fuel reserves there. Had they done that The US fleet would have had to retreat to the US mainland leaving the Japanese unopposed in the Pacific.)
As mentioned before, poor tactical training of officers.
The Klingons seemed to have pretty strong shields too, the fire of 5 torpedoes caused only "moderate damage" to one ship.
There could also have been early big victories for the Klingons, that pushed Starfleet on its heels. A position it couldn't recover from.
Early loss of key facilities near the Klingon boarder. (Think of that like the Japanese attack on Pearl Harbor. The Japanese caused a lot of damage, but failed to destroy the fuel reserves there. Had they done that The US fleet would have had to retreat to the US mainland leaving the Japanese unopposed in the Pacific.)
As mentioned before, poor tactical training of officers.
The Klingons seemed to have pretty strong shields too, the fire of 5 torpedoes caused only "moderate damage" to one ship.
There could also have been early big victories for the Klingons, that pushed Starfleet on its heels. A position it couldn't recover from.