Given the devil is in the details, I will break this down if I may. all of this assumes Canadian force development IOTL until 1914 and US force development IOTL until April 1917; if the US does stuff that may be a greater threat to Canada, both she and Britain will do things in response, making the butterflies flap.
It took the US less than a year to be sending 300,000 men pe month to Europe.
I thought is was a touch over a year, 14 months, but I'm not going to quibble about that. What's important is that these 300,000 went directly to Australian-Canadian-British run training camps to undertake courses lasting at least 6 weeks.
After constant Army reform beginning in 1903, and going all the way to full NG mobilisation on the southern border in June 1916 and the declaration of war in April 1917, by the end of 1917 the US Army only had 4 divisions in France.
1st & 2nd Corps were stood up in January 1918, 3rd Corps in May 1918,........... 7th Corps in August 1918 and 8th & 9th Corps after the Armistice.
1st Army was stood up in June 1918, 2nd Army in September 1918 and 3rd Army after the Armistice.
While the Big Red One fired the first US Shell toward German lines in October 1917 only 4 divisions were battle-ready in Spring 1918.
The first Divisional battles were in May and June 1918 and the first Army level offensive was in September 1918.
So just like Secretary of War Stimson predicted in his June 1914 report, it would take the US more than a year to build an army capable of fighting a high intensity war.
GIven that the Anglo-French forces used more American weapons than America used European weapons
Only small arms. The US had its own range of field artillery leading up to WW1, yet when war came it decided to instead produce British and French designed guns
en masse. However the AEF was wholly equipped with British and French guns and tanks built in British and French factories, no US-manufactured field piece was fired at CP forces in WW1.
Canada is not going to be a threat to America.
I certainly never implied that, my implication was that the correlation of forces in 1914 was such that the US couldn't steamroll Canada. America had 3 regular and 12 NG divisions which have to guard both northern and southern borders, intervene in Latin America about 4 or 5 times
and invade Canada and defeat the equivalent of 3 Canadian divisions.
ignores the very formidable state militias (formidable by North American standards)
From what I understand the reason why the Preparedness Movement kept calling for a Federal reserve army was because the National Guard was so shit. IIUC in 1914 they only had to do a 5 day camp and some voluntary drilling and when mobilised in 1916 (after a couple of years of improvements, NY even formed a heavy artillery btn) they still had to send home quite a lot of amputees and chronically sick men. Also the states didn't build nice, balanced NG units, they tended to focus on the cheaper and more useful infantry and cavalry at the expense of expensive artillery, which isn't ideal in the lead up to WW1.
On top of that I believe there were considerable restrictions on the Federal use of the NG, I couldn't find a reference if asked, but I think that until the 1916 Defence Act it would have been illegal to send them over the border into neighbouring countries. That said, the 1916-17 Federalisation on the Mexican border was a godsend for the US, it shook out so many problems.
Comparing the Canadian militia to the American regular army mobile field force is beyond ridiculous
Why do you say that? Given the limitation of the NG on going onto the offensive, as well as the US' other commitments requiring NG units it will be the 3 US RA divisions going onto the offensive into Canada. These will be met by the 3 division equivalents of the Canadian Army, who will be fighting on the defensive in their homeland. Most likely the US RA will be better than the bulk of the Canadians, but with no massive superiority in numbers there will be no walkover and I'm sure the Canadians will organise an effective defence of their country that will make the US pay for their gains.
Canada is not going to win an arms race or a war with America and isn't going to try either
Of course not, but they're not going to lie down and accept an invasion either and it is ludicrous to think they will. IOTL Canada
did have an Army with several
dozen battalions, despite the US being benign and in world terms a military pygmy. These several dozen battalions will not march into captivity without firing a shot, they will do what they can to repel the invader, hoping that they win or more likely an ally will come to their aid directly or indirectly.