Rambam, Hilchot Shabbat, chapter 2, paragraph 23:
[The following rules apply] when gentiles lay siege to Jewish cities:
If their intent was financial gain, then Shabbat laws
should not be violated because of them,
nor are we allowed to wage war against them.
If a city is located near the border, however,
we should march against them with weapons
and wage war against them, even when
they are [merely] demanding hay or straw.
In any location, if the gentiles’ intent was [to harm] Jewish lives,
or if they engaged in battle with a city or laid siege to it,
without stating a specific intention, we must wage war against them
and the Shabbat laws should be violated because of them.
It is a mitzvah for every member of the Jewish people
who can come [to their assistance] to go out
and help their brothers who are under siege
and save them from the gentiles [even on] Shabbat.
It is forbidden to delay until after Shabbat.
After they [the Jewish soldiers] saved their brothers,
they [the Jewish soldiers] may return home
with their weapons [even] on Shabbat, so that a
dangerous situation will not be created in the future.