In general, travelers using the Visa Waiver Program (VWP) must obtain ESTA approval before entering the United States.
Therefore, if you attempt to travel to the U.S. without ESTA, you may be denied boarding by the airline or refused entry upon arrival.
However, if you hold a valid U.S. visa, you may enter the United States without ESTA.
That said, attempting to travel without ESTA and without a visa is highly impractical and may cause significant disruptions to your travel plans.
For a safe and smooth trip to the United States, it is important to obtain ESTA in advance before departure.
You can easily apply for ESTA through ETAPORTAL, so it is recommended to prepare ahead of time.