Jason here from Shopify Support.
Great question. In most cases, you won't be able to select/edit the country shown in the checkout by default. Shopify uses 4 factors to determine which country shows first in the checkout.
If your store is at point 4, you can edit the default country by editing your available shipping destinations. Remember, it comes out in alphabetical order. This means depending on which shipping zones you have, the country that shows up is most likely going to be the one starts with A, B, or C (or whichever country assorted through the order).
I hope this information helps. However, should you have any questions, please don't hesitate to reply back here and I'd be happy to help out!
All the best,
This is actually pretty crappy and may explain why my US customers are not buying anything: most of my site's visitors are from Europe, where I have to add VAT, so a US-based customer going to checkout is suddenly going to see a big tax where there shouldn't be any just because Shopify defaulted the country to Europe.
Is there a way to just keep the country field blank until the user enters it? Or at least dynamically update the tax when the user changes the country? Right now if you change from a European/VAT country to US the tax is still showing up: the checkout doesn't update to reflect that lack of tax.