Not only is there a negative mitzvah prohibiting us from drafting a new groom during the first year of his marriage (Mitzvah #581), it is actually a positive mitzvah for the groom to spend the first year rejoicing with his wife. Even today, people decline business trips because they're in “shana rishona”–literally, “the first year.” (There is a difference of opinion as to whether the wife's permission in any way impacts the husband's ability to go. It seems to be permissible for the husband to travel for the sake of a mitzvah.)
The reason, as we said in the previous mitzvah, is to help get the happy couple off on the right foot. We don't want him out of town on business for weeks, making his wife feel neglected. As newlyweds, they're supposed to spend time together, so the Torah enables them to do so.
This mitzvah applies in Temple times, when we go out to war. It is discussed in the Talmud in tractate Sotah (43a) and codified in the Mishneh Torah in the seventh chapter of Hilchos Melachim. This mitzvah is #214 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.