For Canada the deal is a big deal because the British/Americans/Australians are building something they need without them. I am sure if the Canadians get a different PM and wish to do a similar deal they could. The Canadians are in NAFTA, they are part of the wider US command in North American, they are economically integrated with the US and militarily protected by it. Its like Ireland's relationship with Britain.
The issue for Canada is that being economically dependent on the US is one thing, but being both economically and militarily dependent is totally different. The British are learning this, as we are basically militarily dependent on the Americans and because we left the EU, we will increasingly becoming economically dependent on them. Its basically a form of being consumed by the larger Empire, and you then have no politically or cultural control over your own destiny in the world. The British could still find a way out, we have some limited but good options. The Canadians don't have options, they had Britain and that was it. So if the Canadians want to break out and have independence have political maneuver, they would need a much more powerful military. Countries which are American allies, but refuse to support the American wars are usually very politically insecure countries. Britain refused during the Viet Nam war, refused again in Syria in 2014. People think its national independence, but its just political posturing really.
The Canadians have a very capable, professional, small military. That's about all there is to it. The Australians have a very professional, capable, small military. The difference is the Australians have to deal with being thousands of miles from American protection, the Canadians don't. So like Ireland who knows the British must protect them, you know the Americans must protect you. You do matter greatly to the Americans, they respect you and your independence. However any attempt by you to break out of this partnership will be seen by the Americans as a threat. They don't actually want you spending way more on defence and having a powerful, large, defensive military force. That is actually a threat to the US homeland. Its the same way the Americans will interfere in Ireland and direct British affairs, they don't want us to be an independent military power because we invaded the US twice. The US minds much less if India/Australia have independent policies and capabilities because they are further away. Even Japan doesn't have an independent military, its all just copies of American stuff. For all these nation having weakened military's and economies, the American have enforced a Pax since WW2.
