Answer;
The United States has troops in place to help patrol the demilitarized zone between North Korea and South Korea.
Step-by-step explanation;
Up until the end of World War II, Korea had been a single political entity governing the Korean Peninsula, when the Soviet Union and United States each occupied the northern and southern halves respectively.
This division resulted to the founding of modern day North Korea and South Korea. Tensions between the two countries remain high as both countries each has an intention of bringing a unified peninsula under its own rule.
The United States has troops are in place to help patrol the demilitarized zone between North Korea and South Korea, this best describes the US involvement in the Korean Peninsula today.