Teller immediately wrote another letter touting the success of the concept. This time he wrote to Paul Nitze, the head negotiator of START, and Robert McFarlane, head of the US National Security Council. Nitze was about to begin negotiations on the START arms limitations talks. Teller said Super-Excalibur would be so powerful the US should not seriously negotiate on any sort of even footing and that the talks should be delayed because they included limits or outright bans on underground testing, which would make further work on Super-Excalibur almost impossible.