Sub space weapons
From FEInfobase
Prior to the outbreak of the war the use of sub space weapons was banned by treaties with every major power in the Alpha and Beta Quadrants, to use them at all was considered as bad as murdering 10,000 men. When it became apparent that there were no other weapons available which were able to harm the Borg quickly enough to stop their onslaught several Federation vessels were loaded with Galatea weapons supplied by the Cardassian Union. There was considerable discussion about the weapons use within the Federation Council before the order to deploy them, several prominant members felt that the end did not justify the means and that using the weapons against the Borg was no better than using them against anyone else. Other hardliners felt differently, not using every means at the Federation's disposal to remove the Borg threat was tantamount to treason against the Federation as a whole.
The hardliners won out and in 2385 the first attacks on the Borg Cubes using sub space weapons were launched. The damage inflicted was devesating the sheer random way in which the weapons affected matter proved difficult for the Borg to adapt too, this sucess put a stop to any discussion within the council as to the legitimacy of the weapons use.