Aﬀected individuals were less likely to volunteer in WWII and more likely to marry within their ethnic group and to choose decidedly German names for their oﬀspring. Rather than facilitating the assimilation of immigrant children, the policy instigated a backlash, heightening the sense of cultural identity among the minority...
Apparently it went further than this, with some states even banning the use of German over the telephone and some language that wouldn’t look too out of place in some contemporary discourse:
A 1915 pamphlet of the American Defense League, one of the largest nationalist political groups of the time, reads as follows: “Any language which produces a people of ruthless conquistadores [sic] such as now exists in Germany, is not ﬁt to teach clean and pure American boys and girls.”
But the main thing is, the heightening of minority identity is similar, despite it being a completely different country and affected group:
In line with the model, the backlash is greater in counties with a smaller share of German population. This is consistent with a cultural transmission mechanism in which parental and peer socialization are substitutes: In places where Germans constitute a smaller minority, parents try harder to shape each child’s sense of ethnicity because they cannot reasonably expect that children will be socialized in their ethnic culture through peer interaction alone… The extent of the backlash was higher also in counties with a greater share of Lutherans.. The implication is that communities with a greater initial sense of ethnic identity reacted more adversely to assimilation policies… The number of pupils enrolled in Sunday schools increased post-war in states that experienced a German language ban. No corresponding increase was observed in other activities of the church, such as number of schools or services held in German. This suggests that the backlash was driven by increased demand of parents for German enculturation, and not by increased supply of ethnic indoctrination by the church.
As always my emphasis.