What will NATO do if Putin uses nuclear, biological or chemical weapons?

As long as Putin does not use weapons of mass destruction, even "tactical" ones, the reasoning remains valid that we should not have NATO troops fight in the war, so as to avoid direct fighting between NATO and Putin, since that fighting could turn nuclear.

However, that reason will lose its validity if Putin does use them. To deter him from using them, we should make it clear that his use of weapons of mass destruction could trigger NATO's use of conventional weapons against the Putin forces and Russia's military power, at least for a while.

After a week or two of that fighting, it would make sense to give him a chance to deescalate — offering to pull the NATO troops out of the fighting if Putin commits to ceasing use of weapons of mass destruction.