"The Zimmermann telegram was decoded by Room 40 of British naval intelligence and planted just at the right moment to make sure that Woodrow Wilson would blow his top.
"When Wilson saw the extent to which Germany was prepared to violate the borders of the United States, there was no choice in his mind but to say that there was a moral issue of vital interest to the United States.
"And it's the mixture of the two together – the right triumphant over evil and the national boundaries of the United States – that made it almost impossible for a president (once this had been made public) to stand back and say: 'We're warning you, again. If you do this one more time – if you ask for California, Oregon and Washington – then we'll really get angry.'
"But it was a point at which American opinion could no longer tolerate the clear violation of what anyone in power – Republican or Democrat, it didn't even matter – what anyone in power would say, were vital American national interests."