How to select state on country bases in drupal webform
Use the Webform Simple Hierarchical Select and the Simple hierarchical select modules; just create a vocabulary and add countries and states hierarchically.

If you don't have vocabularies for countries and states, using the Taxonomy Manager module helps you to import countries and states from a simple list like the following. 

United Stated
- Alabama
- Washington
Iran
- Tehran
India
- Mbambi