Ukraine

Why should repairing vehicles be any more of a casus belli, then providing it in the first place. Or things like providing intel or training soldiers? The UK for example is providing long range storm shadow cruise missiles that can be programmed to target a specific objective, presumably training how to use it, and maybe even intel that leads to target selection. At that point one could argue that the only major difference is who presses the button.

I think it boils down to the fact that there is no strict definition what counts as a cases belli, but it rather is a choice whether or not someone declares it one. If they wanted to I am sure both sides Russia and NATO could declare a number of instances as such, but they choose not to, since neither party is interested in a direct confrontation.

Because that would have serious consequences which might even come down to nuclear weapons, considering the imbalance of strength in conventional military. With NATO (especially the US) vastly outclassing Russia.