To ilustrate the scenario with maps:
North America, 1914 (before the first world war started in North America:
After the Canadian counter offensive and the first confederate offensives which took control of southern Missouri as well as parts of southern Kentucky and a Kansas:
Btw during the scarce three months that the Canadian ocupation lasted on the state of Washington, a decree was sanctioned by the Canadian Governor General in virtue of which the state of Washington name was renamed as "George III Land".
After the US summer offensives which liberated the northern frontier:
And that's how the frontier got stuck until the end of world war 1.
What happened with Alaska, you may ask? Alaska wasn't seen as an important objective and neither Canada wanted to invade it and the US had not enough resources to plan an invasion of western Canada from there.
May I ask when and how Britain lost British Honduras, Jamaica (Home to the RN Caribbean Fleet) and Newfoundland?