Defence Scheme No. 1 was created on April 12, 1921 and details a surprise invasion of the northern United States as soon as possible after evidence was received of an American invasion of Canada

And of course its US equivalent War Plan Red:

The war plan outlined those actions that would be necessary to initiate war between Great Britain and the United States. The plan suggested that the British would initially have the upper hand by virtue of the strength of its navy. The plan further assumed that Britain would probably use its dominion in Canada as a springboard from which to initiate a retaliatory invasion of the United States. The assumption was taken that at first Britain would fight a defensive battle against invading American forces, but that the US would eventually defeat the British by blockading the United Kingdom and economically isolating it.^[3]