This line of thinking is very limited and myopic, Diana. Imagine you are a group that doesn’t have a ‘main attack’ planned. You’re a small group, a few thousand at best. You don’t have the wherewithal to challenge any significant military force directly. There is no way you can take and hold a strategic objective, no way you can prosecute a conventional war without being utterly annhilated, and quickly. So you take to asymmetric warfare.
There are ways to prosecute an asymmetric war without verging into terrorism: restrict your target list to valid military targets, make your actions, and affiliations known at the initiation of hostilities, and (and this is important and required) make a legitimate effort to remove civilians and non-combatants from danger, and minimize the collateral damage inflicted upon them.
The classic example is an armed party boards a civilian transport. Asking everyone to remain calm, they tell the passengers they are member of an insurgent milita, giving the name of the militia. They then explain that there are members of the armed forces of the oppressor nation they are at war with on the transport, and ask those members to identify themselves and surrender to ensure no legitimate civilians are harmed. These steps are required in order to engage in legitimate, ‘legal’, asymmetric warfare that does not involve war crimes.
Failure to take these minimal actions—identifying the attacking party by means of announcement, uniform, or other livery (like your ident transponder that says you’re part of CalMil), and making an effort to avoid civilian loss of life—is considered indicative of a desire to sow fear among the civilian populace, and the attack is considered terrorism, and a war crime.
And that’s true whether the attack is shooting soldiers on a transport, or detonating an AM charge that annihilates a Navy space station. The scale of the attack has nothing to do with the legitimacy of the action.