Easy, have the 'soldiers' wearing British or French uniforms with 'heroic German passengers' able to help save them. Or have the 'soldiers' actually be drunkards or 'spies' with forged evidence of plans to attack key American infrastructure.
A sort of reverse Zimmermann telegram might be plausible too, perhaps official-looking paperwork is found along with a map of 'Future Canadian (or British and/or French) Territories and Provinces' that are to be spared or where damage is to be minimized to it can be 'taken intact'. These might include any combination of the following: Upper Peninsula of Michigan, Maine/New Ireland, Montana, North Dakota, northern Idaho, northern New York State (new border at the Erie Canal?), northern Minnesota, Vermont, New Hampshire, Hawaii, Philippines, Cuba, Panama Canal, City of New Orleans, Florida, etc.
A sort of reverse Zimmermann telegram might be plausible too, perhaps official-looking paperwork is found along with a map of 'Future Canadian (or British and/or French) Territories and Provinces' that are to be spared or where damage is to be minimized to it can be 'taken intact'. These might include any combination of the following: Upper Peninsula of Michigan, Maine/New Ireland, Montana, North Dakota, northern Idaho, northern New York State (new border at the Erie Canal?), northern Minnesota, Vermont, New Hampshire, Hawaii, Philippines, Cuba, Panama Canal, City of New Orleans, Florida, etc.