Putin has warned the west there are lines NOT to cross or he may...

Putin has warned the west there are lines NOT to cross or he may use tactical nukes - Biden has been cautious to seek assurances from Ukraine no attack will be made on Russian soil for example
But the flip side of that is if Putin WAS to use these sorts of weapons he risks the west being forced to put boots on the ground in Ukraine .

then if the west was pushed too hard , there is Kaliningrad and if NATO took it in retaliation for Putin excesses there is nothing Putin would be able to do to stop

not just military considerations here , both sides are constrained by possible future threats