We Never Know
No Slack
We have a professional army which is more than enough to deter a Russian invading force (not a nuclear attack but what good do guns do in a nuclear launch?).
As for China they have enough people in their country that they would probably defeat us in an invasion, but at the expense of having Australian coal and uranium supply cut until they could successfully commandeer it which would probably cause them enough casualties and unrest back home to make war unfeasible for them anyway.
In my opinion.
How would Ukraine be doing without the US sending them guns/weapons for their army and civilians?