When Mars and Pluto are in contact in the birth chart, we had a fair amount of spiritual warfare going on within the person.
Pluto is the subconscious desire; it's compulsive, obsessional, subconscious, all encompassing and comes with a hefty dose of survival instinct. Pluto destroys first, and then rebuilds. On the other hand, Mars - the lower octave of Pluto- is our conscious desire, what we want, what we strive for, how we go about the things we want. Mars is also concerned with survival, but more so, Mars wants to win and dominate. To Mars, there are only winners or losers, and if you're not winning - guess what you're doing?
We have a few situations possible with Mars and Pluto in contact. The first option is when the contact is harmonious, which includes the conjunction, trine, and sextile. When the contact is harmonious, meaning that there is no conflict between the subconscious desires and the conscious desires, we have someone who is quite literally unstoppable. There is a primal, predatory energy about Pluto - Mars which nothing can stop. Once a goal is set, anything standing in the way will be overcome, burnt, destroyed, until one reaches its goal. There is most definitely a certain degree of ruthlessness that comes with this aspect, which is why I started the article by mentioning an innate spiritual warfare associated with this aspect. Pluto - Mars can definitely be an indication of a tendency to be self-serving, callous, predatory, lacking in empathy. It can be a key feature in charts of high machiavellian & psychopaths, but it is of course not a curse nor a generalization one can make. The potential is there, the rest of the chart and the level of consciousness of the person will determine how this aspect is used.
When the aspect is harmonious (again - I include the conjunction, as well as trine and sextile) there is nothing standing in the way of the person. Their goals WILL be achieved, the only question is when and how, but it will. There is nothing stopping them, and there can be an obsession, a compulsion, to achieve goals. The problem is - as with every obsession - it can become all encompassing, all consuming, and eat out other parts of their life, until imbalances are reached. This is of course much truer of the conjunction than any other harmonious aspect.
Pluto - Mars of course has a hefty dose of sexuality associated with it - and again here we have two options. Mars sextile Pluto and Mars trine Pluto are much more likely to have a fairly balanced relationship to sexuality, but there is also the potential, and much more so with Mars conjunct Pluto, to become obsessed or consumed by sex. Sexuality can be linked with owning someone else, possessing, but also - bonding with someone else on a very primal, and soul level. Sex is meant to be an experience of death and rebirth, and they can be quite mesmerizing to anyone who doesn't have such a strong Plutonian energy.
Sexually, Mars and Pluto are associated with a fair amount of violence and domination, which can be simply a matter of taste and a dominant/submissive kind of dynamic, but unfortunately, it does have a symbolic of sexual abuse. In the mythology of it, Pluto - the lord of hell- abducts Persephone and bring her to the underworld, where she is raped and forced to become the Queen of the underworld. The myth is supposed to represent how one's innocence and virginity is taken by force, an initiation rite leading to empowerment. It's quite the tale, and unfortunately it does sometimes translate in actual sexual abuse, whether the person is on the Pluto side, or the persephone side.
Lastly, and this is more so true of Mars conjunct Pluto and the below difficult aspects, Mars - Pluto can be utterly violent, psychologically or physically. As Pluto is obsessive, compulsive, and quite boundless / unleashed, when it meets Mars, the planet of war and aggression, there can be a very instinctive instinct of protecting, defending, conquering, and dominating. This gives men with very "masculine energy" in the primal sense of it, but it also give people - men or women- with a strong inclination for violence and aggression. Mars - Pluto is linked to high criminality and as I mentioned above, it can also be linked with Dark Triad traits, due to mostly three factors: The very strong instinct of survival, the very strong instinct to dominate, and the tendency to have such a tunnel vision that nothing can stand in the way of the person; not even morality or the law. To gauge whether someone would be stopped by morals or the law, you'd want to look at their Jupiter, among other things, to see where their moral compass stands.
When Mars is in difficult aspect to Pluto, like for example Mars square Pluto, Mars opposite Pluto, or Mars quincunx Pluto, everything we just said above is applicable, but there is an additional element to it.
Here, we have the conscious desires and drive (Mars), which are coming in the way of the soul desires (Pluto). The classic representation of this is someone who get caught up in chasing goals and dreams with a revenge. The energy is relentlessly dedicated to a goal, with complete and utter tunnel vision, but once the goal is achieved - the person is left feeling unfulfilled. Something is missing, it's not giving them the hit, the high, that they were hoping for. The person feels let down, but soon enough - they set their eyes on a new prize, and go at it with passion and relentlessness. Yet again, the cycle repeats.
This is particularly true the quincunx and opposition of Mars and Pluto, but here the matter is to learn how to overcome the fleeting desires, whether they're professional, sexual, romantic, or else - and learn how to only pursue the things that are truly aligned with your soul and purpose. Some spiritual work needs to be done, otherwise you end up pursuing endlessly things and goals which aren't aligned with you and therefore not fulfilling, and you suffer a bad case of new shiny object syndrome.
Recommended reading for Mars - Pluto:
"Pluto: The evolutionary journey of the soul": By Jeff Green. A wonderful and in depth take on Pluto
"The book of Pluto": By Steven Forrest. Also a wonderful read.
"Healing Pluto problems": A take on healing Pluto
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