China claims almost all of the South China Sea, including islands more than 800 miles from the Chinese mainland, despite objections from neighbors such as the Philippines, Malaysia, Brunei and Vietnam.

Beijing has also created artificial islands in the area, outfitting some of them with military features. According to the US, China has reclaimed more than 3,000 acres in the Spratly Islands since 2014.

http://edition.cnn.com/2017/02/19/us/us-carrier-south-china-sea/

Why is China making islands, and building military bases? To claim/control the South China Sea

Why is there a conflict regarding the South China Sea? China believes they have the historical right to own the South China Sea, however the UN law of the sea states that countries have the right to claim their territorial waters (200 miles off their shore). The problem is that the 9 dash line and the exclusive economic zones overlap, meaning that several countries are claiming to have the right over the same areas. 

How many countries lay claim to the South China Sea? 5 countries (China, Vietnam, the Philippines, Malaysia and Brunei)

Why is the South China Sea so important? Incredibly rich in natural recourses (oil and fish)

Why is the US involved in this conflict? To protect international waters, and to make sure China does not have too much power in the South China Sea. 

How is the US responding to China’s new military islands? They use their large navy base, sending ships to the South China Sea)