Every country has its own laws regarding entrance by non-citizens. Please check the specific laws governing the country you are visiting. It is your sole responsibility to obtain all necessary documentation prior to your departure. Regardless of the ticket you purchase, applicable immigration laws apply.
Please note that there is a new United States regulation in effect that requires citizens from some countries to obtain a visa if their flight is stopping over in the US, even if the US is not the final destination.
In addition, citizens from waiver-countries visiting the US should be aware that per the US Department of State, travelers entering the United States under the Visa Waiver Program must have a machine-readable passport. Any traveler without a machine-readable passport will be required to obtain a visa before coming to the United States.
Click for more information on US visa regulations
Please check all appropriate laws for the country you are visiting.