Not only may we not allow the descendants of Ammon and Moab to marry in, when going to war we were not to first extend an offer of peace as was the usual modus operandi (see Mitzvah #527). Again, this is because these nations stabbed the Jews in the back when they left Egypt, rendering themselves undeserving of such considerations.
This mitzvah only applied to enemy combatants, not civilians. As with the previous mitzvah, it is unenforceable today, since Sennacherib, king of Assyria, mixed up the descendants of Ammon and Moab among those of many other nations.
This prohibition is discussed in the Midrash in the Sifre. It is codified in the Mishneh Torah in the sixth chapter of Hilchos Melachim and is #56 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.