Make the Country field default to US.

tw_ee_change_default_ee_country_and_state_option.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action(
 'wp_enqueue_scripts', 
 'tw_ee_change_default_ee_country_and_state_option',  
    20
);
function tw_ee_change_default_ee_country_and_state_option(){
    $custom_js = 'jQuery(document).ready(function($){';
    $custom_js .= '$(".ee-reg-qstn-country").val("US").trigger("change");';
    $custom_js .= '$(".ee-reg-qstn-state").val(17);'; // Illinois
    $custom_js .= '});';
    wp_add_inline_script('ee_form_section_validation', $custom_js);
}

tw_ee_change_default_ee_country_option.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action(
 'wp_enqueue_scripts', 
 'tw_ee_change_default_ee_country_option',  
    20
);
function tw_ee_change_default_ee_country_option(){
    $custom_js = 'jQuery(document).ready(function($){';
    $custom_js .= '$(".ee-reg-qstn-country").val("US").trigger("change");';
    $custom_js .= '});';
    wp_add_inline_script('ee_form_section_validation', $custom_js);
}