This is purely about military doctrine. Much effort has been made to avoid using US-made fighter planes in the Ukrainian theater because that would be seen as "escalation" and lead Russia to try and target the US directly, we are told. However, the supply of Javelins and Stingers direct from the US, are clearly not taken as escalatory. Could someone explain what is different here? Or is it simply that Russia declared terms beforehand that Jet fighters are an escalation, but the rest is OK? Is there some war doctrine governing escalation?