The eternal law of Dharmo Rakshati Rakshita means that if a person upholds dharma then dharma protects the yogi and his karma is simultaneosly preserved.
From karma yoga to dharmayoga one is going along progressively along ones personal truth path without attachments as to the outcome of ones actions. But following the progressive truth path turns karma into dharma which is the highest basisbfor one's actions. It is then that Dharmo Rakshati Rakshita is evident.
This is the inseparable reationship of truth and dharma as the eternal law. For this reason an old trditional Odiya prayer asks God to take the devotee along the path of truth and dharma.
In a civilised place the means justifiesnthe ends; in a jungle the ends justifies the means. So you must know what kind of place you live in. You may have to fight a dharmayudha when prcticing dharmayoga. Or just do nothing when your liberty is safe and assured.
In the battle to survive in independence and liberty as the dharmayudha one does not need to analyse and calculate strategies and tactics on what is a reasonable and proportionate respose to enemy intrusions into ones privacy, it comes naturally wheen one is on the path of truth and dharma, such is the level of awareness in dharmayoga. This is when one has previosly lost all ones attachments through satya-advaita yoga and has becomeba Vishista-advaitist with all the knowledge that one needs.