In the current war with Ukraine it seems like the West can send any forms of weapons (short of nuclear warheads) to Ukraine without causing the conflict to escalate beyond Ukraine’s borders. This includes long range weaponry capable of hitting military targets within Russia’s territory. However it seems like sending actual troops is out of the question as this would cause a significant escalation of the conflict.

Why is this the case? What’s the difference between sending a HIMARS launcher and sending a battalion of troops?