The answer to your question is that if the WN flight arrives anywhere close to on schedule, you are fine. As you are not connecting, but starting a second ticket, consider that so long as you have collected your bags from your WN flight at IAD at 90-120 minutes, you have more time than any international carrier out of IAD requires.
However, the question you have to face is your own risk tolerance if you no show for your international flight. Forget about "flat tires" for international and consider that your tickets will have been cancelled and you must plan on having to purchase new tickets at walk up prices, likely an eye-poppingly expensive proposition. You might find some mercy from somebody, but you cannot count on it and you cannot even count on finding space. In addition, depending on the destination, the next flight might be 1-2 days later and you will be on the hook for hotels and food.
When traveling on separate tickets, you should allow an overnight at IAD. Better yet, check out pricing on pretty much any other carrier as most of them interline with each other and when you connect, your connection is protected, e.g., the late-delivering carrier will rebook you at no charge.
These situations sometimes cannot be avoided. But, you really need to go into them with your eyes open as the cheapest ticket is not necessarily the least expensive journey.