Ianus (Janus), the early Roman god of choices:
His temple doors were closed to signify that the republic, later the empire, was at peace, and left open when the centurians marched off to war.

This may have been confused down the line with Mars and Ares.

Also:
Mars has been explained as a sun-god, a god of wind and storm, a god of the year and a god of vegetation; and he has been compared with Apollo by Roscher (Apollo, and Mars, 1873, and in the article “Mars” in his Lexicon of Mythology). But in historical times his chief function at Rome was to protect the state in war, and it is as a god of war that he is known to all readers of Roman literature. So entirely did this characteristic get the better of all others, that his name came to be used as a synonym for bellum; and in the latest and most careful of all accounts of the Roman religion he is pronounced to have been from first to last a god of war only (see Wissowa, Religion und Kultus der Romer, p. 129 seq.).

Until the time of Augustus, Mars had but two temples at Rome, and both are connected with warlike operations. One of these was originally only an altar; it was in the Campus Martiu~ the exercising-ground of the army. The other was outside the Porta Capena, the gate through which the army marched on its way to campaigns to the south: here too each year the Equites met in order to start in procession through the city (Dion. Hal. 6. 13).
(One Nifty Source: The 1911 Edition Encyclopedia)

the implication here is that the gates to the Mars temple were opened for the troops to depart, as well as for the start of city parades, but does not imply the doors were opened only during times of war, that they remained open, or that they remained closed during times of peace.