Mars in the twelfth house is a bad placement for it. 12th House belongs to Saturn who considers Mars as his enemy. Placement of Mars in twelfth house causes a strong case for Mangal Dosha. There could be jealous friends and well wishers who could work behind the back of native to cause his/her downfall. 

     Mars in twelfth house may give negative publicity at times due to the strong aspect on third house. Native may love sex and bedroom pleasures but there is an acute shortage of them especially if Mars is in a fiery sign like Aries, Sagittarius or Leo and co-tenant with Sun or Ketu.  The sixth house aspect may give diseases like ulcers and burning sensation in the stomach and intestine region. The aspect on seventh house causes frictions with husband/wife and business partners. Natives are advised to be careful in partnership business. If possible they should avoid business partnership. There is a strong case for second marriage especially if both seventh house and eight house contain malefics. There is a possible chance of deception and loss of money if the native is not careful in his financial dealings. There is fear of fire and prison. In some cases there is a possibility of burning or getting locked up in prison. 

     Twelfth house is not the best place for Mars. But a few precautions could keep the natives out of harms way. Native need to be careful of hidden enemies who guise themselves as friends and well wishers. They need to take extra care not to cheat or get cheated in financial dealings. Stay away from business partnership if possible. In marriage native requires to devleop a loyal and compromising attitude to make the marriage sustainable. Prayers to Lord Vishnu and Goddess Laxmi should ease the anger related issues and troubled financial dealings. 

References:

Raman B. V (2016) How to Judge A HOROSCOPE Volume II, Delhi,Delhi: Motilal Banarsidass Publishers

Jyotish Pravesh By Chandrakant Pathak, Surat: Gajanan Publishers, Surat

Bhrigu Samhita