Vietnam War, United States, China, USSR
Tactical nukes could destroy tunnel systems, but required precise targeting. If targeting were that precise why not just use conventional B-52 strikes?
Here's What You Need To Remember: U.S. nuclear strikes in Vietnam might have compelled the USSR and/or China to respond. The Soviet Union could not afford another loss of face only four years after the Cuban Missile Crisis and might well have supplied North Vietnam with tactical nukes.
Read this story at The National Interest