What?

The US has deployed its fifth generation fighter planes over South Korea to participate in annual, Ace 18 war drills. 

When and Where?

The US warplanes have started conducting combat exercises on Monday over the South Korean peninsula. 

Why?

The combat exercises are a part of the annual drill, Ace 18 conducted by the US. However, this exercise is being seen as a show of strength amidst rising escalations with North Korea over the weekend.