The US president has a "Presidential Decision Handbook" available at all times, which includes a menu of nuclear retaliation options. In theory he can launch these on his authority alone. Some would kill more than 100 million people:

"The president can select nuclear strike packages against three categories — military targets, war-supporting or economic targets and leadership targets. There are sub-options, and the menu allows a president to withhold attacks on specific targets.

Two officials said that the “Black Book” also includes estimates on the number of casualties for each of the main options that run into the millions, and in some cases over 100 million. Officials who have dealt with nuclear-war options said that learning the details can be horrifying and that there is a “Dr. Strangelove” feel to the whole enterprise."

https://www.google.com/amp/s/www.washingtonpost.com/amphtml/...

I can see the next president tweeting at 3.30 am: "Should I launch Black Book option 17C? I'll keep you in suspense..."