"Walk not in their ways" - The Threat of Assimilation: Rambam, Shulhan Arukh, and Rav Yehiel Yaakov Weinberg

1) We may not follow the statutes of the idolaters or resemble them in their [style] of dress, coiffure, or the like, as [Leviticus 20:23] states: "Do not follow the statutes of the nation [that I am driving out before you]," as [Leviticus 18:3] states: "Do not follow their statutes," and as [Deuteronomy 12:30] states: "Be careful, lest you inquire after them."
[All these verses] share a single theme: they warn us not to try to resemble [the gentiles]. Instead, the Jews should be separate from them and distinct in their dress and in their deeds, as they are in their ideals and character traits. In this context, [Leviticus 20:26] states: "I have separated you from the nations [to be Mine]."
[Thus,] one may not wear a garment which is unique to them or grow the tresses of our hair as they do. We may not shave our heads from the sides and leave hair in the center as they do. This is called a blorit. We may not shave the hair on the front of our faces from ear to ear and leave a growth at the back of our heads as they do. We may not build Temples in order that many people may enter as they do. Whoever performs one of the above or a deed of this nature is [liable for] lashes.

(1) 1. Not To Dress Like A Non-Jew, One [i.e., a Jew] should not follow the customs of non-Jews (nor should one try to resemble them) (Tur in the name of the Rambam). One should not wear clothing that is particular to them [i.e., their culture]; one should not grow forelocks on one’s head.... one should not build places [i.e., buildings]—like the non-Jews’ temples—so that large groups of people will enter them, like [non-Jews] do.

RAMA: Rather, one [i.e., a Jew] should be distinct from them [i.e., non-Jews] in one’s manner of dress and in all of one’s actions. But all of this [i.e., these restrictions] apply only to things that non-Jews do for the sake of licentiousness. For example, they are accustomed to wearing red clothing, which is official/princely clothing, and other clothing that is similarly immodest. [These restrictions also apply] to things that they are accustomed to doing because of a custom or rule that does not have a[ny underlying] reason, out of concern that [a Jew who does such things will follow the] “ways of the Amorites,” and that it has the blemish of [i.e., is tainted by] idol worship inherited from their ancestors. But things that they are accustomed to doing for a useful purpose—such as their custom for expert doctors to wear particular clothing so that the doctors will be recognized as specialists—one is permitted to wear [such clothing]. (Maharik Shoresh 88) [Italian; d. 1480] Similarly, things that are done out of respect or another reason, it is permitted [for one to do such things]. And therefore they said one may burn [the items of deceased] kings, and there is not in this “the ways of the Amorites.” (R"an, Chapter of the Laws of Non-Jews)

(2) 2. One [i.e., a Jew] who is close [i.e., works closely] with the [non-Jewish] authorities and is required to dress in their clothing and to be similar [in appearance] to them, is permitted in all respects [to do so].