Avodah Zarah 16A

אי אפשר, הכי נמי.

If it were possible to avoid selling produce to non-Jews without incurring their animosity, indeed it would be prohibited to sell them. Since limiting sales to non-Jews to such an extent would cause great harm, it is only prohibited to sell them shields.

איכא דאמרי: תריסין היינו טעמא דלא, דכי שלים זינייהו קטלי בגוייהו. ויש אומרים: מוכרים להם תריסין, דכי שלים זינייהו מערק ערקי. אמר רב נחמן אמר רבה בר אבוה: הלכה כיש אומרים.

There are those who say: With regard to shields, this is the reason that one is not allowed to sell them to non-Jews: As when their use of their weapon is finished in battle, they kill with these shields. And accordingly, the reason that some say in the baraita that one may sell shields to them is because they maintain that this is not a concern, as when their weapon is finished they flee, rather than use their shield as a weapon. Rav Naḥman says that Rabba bar Avuh says: The halakha is in accordance with the opinion cited as: Some say.

אמר רב אדא בר אהבה: אין מוכרין להן עששיות של ברזל, מאי טעמא? משום דחלשי מינייהו כלי זיין. אי הכי, אפילו מרי וחציני נמי! אמר רב זביד: בפרזלא הינדואה. והאידנא דקא מזבנינן, אמר רב אשי: לפרסאי דמגנו עילוון.

Rav Adda bar Ahava says: One may not sell blocks [ashashiot] of iron to non-Jews. What is the reason? It is because they forge weapons from them. The Talmud asks: If so, then even hoes and axes should not be sold to them, as they too can be used to forge weapons. Rav Zevid said in response: The ruling of Rav Adda bar Ahava was stated with regard to Indian iron, which is of a superior quality and used only for crafting weapons. The Talmud clarifies: And as for the fact that nowadays we do sell all weapons, Rav Ashi said: We sell the weapons to the Persians, who protect us.