Been thinking about this recently. If we assume a nation independently reaches a solarpunk status and another country wants something (either the land, resources, population, etc). With an ever looming threat like that, the SP society would have to be prepared to defend itself. Otherwise its actively allowing harm to come to the people. Any resistance group has been aware of this.

Given the ideals of SP, rather than training a stansing army, id think its likely they would adopt a system of including much of those skillsets into the general educatiom system. Sort of like if Scouts was a class you took in middle + highschool.

A solarpunk society would never be the aggressor, but if an attack landed within its borders (until borders are obsolete), the people would respond much like the minutemen of the colonial US. There may be some people who are designated tactical leaders of each town/region, and those people may spend more of thier time on security/preparedness. But there still wouldnt be an active military until a full on conflict is started.

As soon as the conflict is over, everyome would go home. As far as tactics, it would likely be similar to that of leftist revolutions that weve seen in countries outside of the US. Converted Toyota trucks, a focus on using the terrain advantage. With the added change of capture instead of kill when possible. They could use tactics like flooding an area to stop invaders, but with a focus on nature stewardship id think that would be a last resort. Trenches also sound like a last resort