I stand corrected. ATACMS have been there, but Ukraine was not allowed to use them to attack the Russian interior - until now. The disputed territories of Donbass and Luhansk being annexed as part of the Russian Federation is not internationally recognized as they are technically still considered part of Ukraine. Therefore the use of ATACMS in these areas is considered an attack on the Russian occupation of Ukrainian territories.

Putin's red line however involved use of these systems to attack the interior of Russia proper and not their use in the Russian occupied territories. Putin made it very clear that they would consider their use to to attack the Russian interior as a direct attack by the US on Russia. As Putin spelled out, the reason is because operation of these systems requires direct American involvement.

Just because Russia declares these areas to be part of the Russian Federation does not make it so - they are disputed lands as they were part of Ukraine. Both sides of this conflict lay claim to those lands and each side has valid points for their claims. That is why they are at war. A peace agreement between Russia and Ukraine would hopefully settle the land dispute issue if agreed to by both sides in this conflict.