I am planning on going to Japan within a month, and was wondering how security works during a layover. My travel would be Chicago to Detroit to Tokyo, and vice versa, while only using Delta throughout the trip.
Would I have to go through security and have my items checked again during a layover or is it only when I return to the U.S.?
You will almost certainly not go through security in Detroit on your outbound flight.
At US airports, domestic arrivals (such as your flight from Chicago) generally exit into the departure hall, meaning you can walk straight to your next flight to Tokyo without any further formalities.
As you mentioned, this isn’t the case on the way back, as you’ll have to clear immigration, collect and re-check any checked bags, then go through TSA security to get to departures.
As a general rule, in most parts of the world, domestic arrivals deplane either directly into the departures hall, or into an area that allows direct access to the departures hall. ORD or MDW to DTW is a domestic flight (both airports are in the same country) even though the full itinerary is international, thus it is extremely unlikely that you will need to re-clear security at DTW unless you leave the secure area (for example, following the signs for parking instead of transfers).
On the return trip, you will not only need to re-clear security at DTW, but also re-check your bags, because US Customs regulations require you to pass through customs with all of your bags, so there will be a baggage claim area just past entry control, followed by the customs checkpoint, then an area to re-check the bags, and finally a TSA checkpoint. As a general rule though, this is relatively efficient, and unless you have a really short connection (less than about 90 minutes based on experience traveling through DTW myself) you should have more than sufficient time to make your connection.
