I think that that both of you have good points, but missed some nuance. Most US-to-US transactions verify cards using the ZIP code (in this context, called AVS

I think that that both of you have good points, but missed some nuance. Most US-to-US transactions verify cards using the ZIP code (in this context, called AVS or the address verification system) and the card number. Most retailers do verify if the CVC and expiration date are also correct, but international retailers might be surprised that you can enter 12/99 and 000 and still pass due to this oddity (https://support.stripe.com/questions/charge-succeeded-despite-cvc-or-zip-check-mismatch https://support.stripe.com/questions/charge-succeeded-despit...). This is also the reason why US cards are favored by criminals (because the security in them are, to be frank, insufficient). In contrast, European retailers do use the expiry date and CVC to verify the card, and some even take this further with 3DS verification (regardless of the origin of the card). In practice, using a US card in Europe does not really cause problems except a) when the retailer only accepts locally-issued cards (or EU cards only as may be the case), b) when your bank does not support 3DS, or c) you did not set a PIN code / retrieve your PIN code from your bank.