The foundation of any enduring society lies in the stability of the family unit. To ensure the strength of a new marriage, a newlywed man is granted an extraordinary privilege: a complete detachment from national and military obligations so he can focus entirely on building his home.
The primary approach among commentators is that this law applies to any woman who is new to the man, including a widow or a divorcee marrying him for the first time. The only exception is a man remarrying his own former wife, as she is not new to him [רש״י, אבן עזרא, תורה תמימה, הירש]. This military exemption, however, is strictly limited to discretionary conflicts. In the event of an obligatory war, where the very survival of the nation hangs in the balance, it is agreed that even a newlywed must leave his home and join the battle [רלב״ג, אברבנאל, ביאור יש״ר].
During standard times of conflict, the newlywed's exemption extends far beyond actual combat. There is a sharp contrast between a newlywed and others who are excused from the front lines, such as a man who is merely engaged or one who has built a house but not yet dedicated it. While those individuals must still contribute to the war effort from the rear by supplying food and water or repairing roads, a married man is entirely freed from all logistical support [רש״י, רמב״ן, מזרחי, אברבנאל]. This total release from public burden also covers civil duties, including taxes, forced labor, and guarding the city walls [אבן עזרא, בכור שור, הירש]. He is meant to be completely free and unburdened [אבן עזרא, קונטרס חיבה יתירה]. Importantly, this sweeping exemption takes effect only after the actual marriage has taken place, not during the engagement period [ביאור יש״ר].
Furthermore, the restriction is directed at the king and military commanders, forbidding them from drafting the man; if he chooses to volunteer of his own free will, he is permitted to do so [העמק דבר]. By extension, this absolute year-long exemption is also applied to individuals in their first year of dedicating a new home or a new vineyard [רש״י, רא״ש, מזרחי].
The specific timeframe of twelve months is rooted in human nature. It is the natural period required to establish a deep emotional connection, take root in love, and adjust to a shared life before the everyday burdens and the responsibilities of raising children begin [תורה תמימה, אברבנאל, הירש]. During this time, the husband has an active duty to bring joy to his wife, rather than merely being happy alongside her. The goal of this profound dedication is to win her heart and instill a deep sense of love and security, thereby preventing future emotional distance or divorce [רש״י, נתינה לגר, רלב״ג, אברבנאל].
On a spiritual level, marriage represents such a profound new beginning that all of a person's past sins are forgiven, leaving him entirely clean [קיצור בעל הטורים]. This dedicated first year mirrors the historical moment when God dwelt with the Israelites at Mount Sinai for a year following the giving of the Torah, much like a groom rejoicing with his bride [רבנו בחיי]. Finally, this period serves as a powerful metaphor for personal transformation. A person who repents and takes on a new soul must dedicate a season to detach from societal distractions. By freeing himself from material pursuits, he can devote his energy with joy and enthusiasm to building his new spiritual world [חתם סופר].