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They considered the Dutch, all Brits, Scandinavians, and northern germans to be of similar enough stock to be in the US. They gave the French and the various groups of Spaniards in American territory a pass too, but considered them different.

It's not that they hated every other group of europeans. Quite the contrary, actually. They were inspired by the romans and the Greeks, they were helped extensively by the French and the Spanish, and they never said anything hostile about anyone south of the Rhine.

They just saw the conflict in Europe at the time, and wanted a US that was cohesive and springing from the same (at least similar) stock that colonized it in the first place, to avoid having the nation fall apart due to ethnic and cultural issues.

History has halfway vindicated them. Opening up the borders to other European groups hasn't proved to be a problem at all. Besides jews from eastern europe, but thats a different story altogether. The only major issue caused by other whites was the Italian mob, which was actually mostly jewish at the top. I dont think having the borders open to all of europe in the first place would have been a big issue. Opening up the borders to people *outside of europe* though, well...