Do you understand how WWI became a world war? When two countries had a conflict between them, a network of alliances caused others countries to become parties to the conflict. That's exactly what's happening here.

The US, through NATO, was deploying lethal capabilities in Ukraine. Russia determined that this was strategically threatening its security, a position that it has held for 30 years and has been acknowledged by US leaders, diplomats, and generals, as well as world leaders and even the leaders of NATO. Russia launched a conflict with Ukraine, and Ukraine only, in order to address its security concern.

If the USA enters the war, unprovoked by Russia, then it would be the USA starting WW3, just like in WW1 a conflict between two countries expanded to include uninvolved parties when they made themselves involved. The problem with the ATACMS is that it requires US/NATO to operate. Within the borders of Ukraine, that means that US forces are killing Russian soldiers in Ukranian territory. While problematic, Russia has only escalated its rhetoric based on this involvement. However, if US/NATO troops were to use ATACMS to strike Russian targets on Russian territory, that would be an act of war against Russia which would require a response. In this way, the US would expanding the war beyond the conflict of Ukraine and Russia to now be Ukraine, Russia, and the US - an escalation to world war.