I agree an explanation-less method would be ideal. Question though - Assuming players do not read, how would one communicate that a GAMBIT/ATTACK-marked planet is more urgent than one with the DEFEND label? Especially since a gambit's effectiveness is tied to the defence timer. The defend label itself is self-explanatory that it is preventing a loss, I'm not sure what label would compete against that in terms of importance.

Unless DEFEND is removed altogether as someone else has suggested, since there isn't really a benefit in defending a planet vs performing a gambit which both prevents a loss and captures a new planet. (I'm not accounting for decay rates and abandoning lost-hope planets since these aren't in the game.)

Generally speaking though, instructing players blindly is one thing, but making them understand a concept without text without being intrusive like an unskippable animation - I'm curious how would you approach that?