JFK airport 1h40 Layover with Delta

The bad news:

• If your incoming flight is severely delayed, there’s no later flight to Nice, so you will either be rerouted via another airport or will have to wait the next day, so the delay may be a bit longer than in other circumstances.
• 1h40 is definitely enough for a normal connection, actually has quite a bit of margin, but not enough for the longer delays (over an hour or so)
• Your flight being operated by a US airline, in this direction you won’t be covered by the EU’s strong protections for passengers (EC261) which would have granted you compensation (several hundred euros) for severe delays in many cases (not all).

The good news:

• In normal circumstances this is more than enough
• There’s no exit passport control in the US
• Since both flights are on the same ticket, in this direction your luggage will be checked through to the final destination, you won’t see it, no need to wait for it or re-check it
• You probably won’t have to go through security, but I’m not familiar enough with JFK to say for sure
• The airline is selling the connection, so they’re quite confident you’ll make it (because there are quite a few strings attached for them if you don’t)
• If you don’t make it because the first flight is delayed, it is the airline’s responsibility to rebook you and take care of you (paying for hotels if you need to stay overnight for instance)
• 1h40 should provide for enough buffer for a delay of up to an hour on the arrival of the incoming flight, possibly more.
• There is usually a bit of margin on transatlantic flights, so they can often (but definitely not always) wait a bit for delayed connecting passengers
• There are probably quite a few alternative routes via Paris, London or other places so you probably would be delayed a few hours rather than a full day in case you miss the second flight
• Delta is a partner of Air France so that gives quite a bit of additional flexibility

All in all, with both flights on a single ticket I wouldn’t even think about a problem beyond a possible delay as in all air travel, so unless you have a strict imperative on arrival in Nice you can just let Delta worry about things for you.

Because both flights' tickets were issued under one PNR, the ticketing (and operating) air carriers are confident that the connection can be made.

If a one-PNR two-flight itinerary fails because the traveler is too late for the second flight, the airline is responsible to get the traveler to the destination, to care for the traveler on the way (food and lodging, if the delay is overnight), and may be liable for monetary damages as well. This is enough pain for airlines to be very careful to sell only ticket pairs with connections the airlines are confident can be made.

No one knows for sure what'll happen on your trip, but the smart money says you'll be fine.

(Text taken from my answer to this question.)

Note that here's also pain and dislocation for the traveler if the connection fails, even if the airline is responsible. And connections do sometimes fail.

For itineraries (like international flights) where missing a flight is a big deal, a conservative approach is to buy both legs' tickets as one PNR (you did this), and in addition seek more connection time. In this itinerary, you could have increased the connection time with an earlier flight from Denver to JFK, even one the day before. There are many more flights from Denver > JFK then there are flights from JFK > Nice. (Traveling a day earlier would make the connection to the second flight more certain, at the cost of a night's lodging and more time.)