If you change flight in the US you need a permission called ESTA. I faced this fact only when I was not allowed to check in on the airport.
You can apply for ESTA online filling in a form with all of your details except for your dog's name. First I starts to fill in an other form by mistake and I was surprised why it insists on my address to stay in the US. After a while it was said that I do not meet a 'transit visa' only an ESTA. Than I started to collect all of the required info: my id number, the address of my last employs, etc.
When I finished even with paying the administration fee of $14 it was 3:30 pm. Officially my flight for Chicago should had set off at 3:00 pm. Fortunately and unfortunately it has a delay of at least 2 hours. So I was able to check in with the kind help of the friendly stuff that I appreciate but in return I will miss my flight to Europe so I have to spend almost a whole day in Chicago.
But I am on my way with full of beautiful experiences and colorful pictures.