The Alignment used advanced Mesan nanotechnology inserted into a person's body in order to influence some basic behavior of the host.
The actual attack was done through William Tyler, a representative of New Age Pharmaceutical in April 1921 PD. The nanites forced him to release a binary neurotoxin disguised as a perfume sample as soon as he saw Queen Berry at a reception. The Treecat Genghis detected the attack, giving a few seconds warning.
The incredibly quick actions of Berry's guard Lara saved the queen, but Lara herself, and everyone else in the room at the time of the attack had been killed.
The attack, while failing to kill Queen Berry, had the intended effect of disrupting peace talks between Haven and Manticore. Enraged, Queen Elizabeth III Winton ordered resumption of hostilities. (HH11)