I forget, what is it called when you claim to be negotiating and then suddenly attack the other side while at the conference table.

Perfidy, it's called Perfidy.

Perfidy (Military/Diplomatic): In international law, this is the specific term for acting in bad faith, such as using a flag of truce or diplomatic negotiations to lure an opponent into a trap before attacking.

Bad Faith Negotiator: A term for someone who never intends to reach a fair agreement, but uses the negotiation process to delay or catch the other side off guard.

Betrayal/Treachery: General terms for the violation of trust involved when one party suddenly attacks after claiming to negotiate.

In contemporary diplomacy, this has been described as "abusing diplomacy to cover up a military operation"