If you’re traveling to the US from Europe, you can enter without a prearranged visa via the Visa Waiver Program.
However, if you plan to do so, then it’s worth noting that US authorities reserve the right to put you in shackles and throw you in a jail for 10 days, based on nothing more than a "hunch".
This is not a theoretical right, it’s one they’ve happily made use of.
It’s Friday, so I decided to walk to Baby Greens to get lunch, as I often do at the end of the week. As I walked, I kept an eye out for fallen pecans to take home and leave out for our squirrels.
I saw one at the end of the street, and picked it up. As I did so, I heard a noise. I looked up, and a squirrel was sitting in the pecan tree, barking at me for stealing his nut. I tossed it back on the ground. He stopped and stared at me. I apologized and walked on.