The constellations that astronomers use to identify parts of the sky are not the same as the Zodiac signs used by astrologers. The Zodiac signs were identical to the constellations 2000 years ago when their modern names were first used. The Signs are an astrological convenience ignoring the differences between two different types of solar calendar. This is described further in The Calendar.
The astrological signs are about a month out from the astronomical constellations. When an astrologer says the Sun is in Cancer, an astronomer will look for it in Gemini.
If Mars is in Virgo, astronomically it means that, if you look at the group of stars known to the Western cultures as Virgo, you will see an extra red star-like object amongst those stars. That will be Mars. However when an astrologer says that Mars in is Virgo, he is talking about the abstract astrological sign that has no physical significance.
Mars will actually be in Leo. The difference in the two solar calendars is caused by an astronomical phenomenon called precession.
An astrologer would make predictions based on the position of Mars in an imaginary sign. An astronomer would be able to point out Mars; it would be found amongst the stars of Leo.
Even to say Mars is IN Leo is incorrect. Mars would be in the same direction as the stars of Leo. These are themselves at different distances. It is a bit like holding a hand against a building saying the hand is IN the the building.
Mars is said to be the god of war because its red colour reminded the ancients of blood. Mars is red because its surface is covered with iron oxides - rust! Also, the constellations in use by the West are not the same as those used...