Magento – How to set up a tax rate for all countries

The following code will:

1. Load a customer address and get the country id e.g. "US" or "DE"
2. Load a collection of all tax rates for that given country

Note that $customer_id should be a relevant customer id and you could use the current logged in customer. Also note that getDefaultBillingAddress is not always set for a customer.

$oCustomer = Mage::getModel('customer/customer')->load($customer_id);
$oAddress = $oCustomer->getDefaultBillingAddress();

/* @var $oTaxRateCollection Mage_Tax_Model_Resource_Calculation_Rate_Collection */
$oTaxRateCollection = Mage::getResourceModel('tax/calculation_rate_collection');
$oTaxRateCollection->addFieldToFilter('tax_country_id', $oAddress->getCountryId());

The easiest way to get a list of all country codes is by directly accessing / exporting the data in the 'directory_country' database table if you have access. This lists all countries with both their ISO 2 and ISO 3 codes.

Note: the above is valid for both Magento 1.x and 2.x and as you have not specified I don't want to presume.

I've also created a gist with the output from the table if it helps: https://gist.github.com/johnhughes1984/e749cee0e3713d49dd90ebe9b305858b