China is aggressively laying claim to most of the South China Sea. What's at stake? Here's everything you need to know:

What is China claiming?
China has issued an audacious map claiming as its territory practically the entire South China Sea and all its islands. The delineated area reaches hundreds of miles from the Chinese mainland, nearly all the way to the coasts of Vietnam, Malaysia, and the Philippines. Under maritime law, each country claims the sea 12 nautical miles out from its coastline as its own territory, and it can also claim an economic zone 200 miles out, within which it can control fishing and mining rights. China is leaving its neighbors only their 12 miles and claiming the rest. It says, for example, that it owns the Spratly Islands. That archipelago is also claimed by the Philippines, Taiwan, and Vietnam — and unlike China, those countries have each established their claim with an airport and a small military outpost in the otherwise uninhabited islands. To remedy that, China is building its own islands in the Spratlys. It says these artificial islands are for "peaceful civilian purposes," but there are indications it intends to establish military bases on them.

How is it making islands?
China has built and deployed gigantic dredgers the size of skyscrapers that can move more than 4,000 cubic meters of sand and rock every hour. The vessels are dredging out deepwater harbors suitable for large ships and dumping massive amounts of sand onto reefs to build islands big enough for military bases. More than 2,000 acres of new land have been formed, mostly in the past six months. China "is not wasting any time trying to get these islands created, infrastructure built, and populated," said James Hardy of Jane's Defence Weekly. One of the seven new islands is almost two miles long and sports a nearly completed airstrip suitable for fighter jets.

Is this legal?
That's a complicated issue. Maritime law does not recognize manmade islands as territory giving a nation sovereignty over waters or airspace. China does have some historic claim to parts of the sea, but then so do many other countries. Brunei, Malaysia, Taiwan, the Philippines, and Vietnam have all used or occupied some of the shoals and islands at various periods in history, and all have submitted legal claims at the United Nations. Yet China refuses to submit to U.N. arbitration. Instead, it is moving ahead, taking existing islands or building new ones, with military hardware expected to come next. "The fear is that China will turn the sea into a Chinese lake," says Rajeev Ranjan Chaturvedy of the Institute of South Asian Studies in Singapore.

Why is the region so important?
The South China Sea is flush with resources and has great strategic significance. The region's fisheries provide food and jobs for millions of people in the surrounding countries. Vast, untapped oil and natural gas reserves are believed to lie undersea. Even more importantly, the sea encompasses key global shipping lanes and sees five times more oil tanker traffic than the Panama Canal. Five of the world's 10 busiest shipping ports are located there. Were China to control access to the sea, it could disrupt oil and trade shipments to all of East and Southeast Asia, paralyzing these nations' economies.

How is China asserting its claim?
Over the past three years, China has been occupying the Scarborough Shoal, a triangle of reefs in the South China Sea long claimed by the Philippines. The two countries had a standoff over fishing rights, and after U.S. mediation, both agreed to withdraw their warships. But China reneged on the deal, and the shoal is now patrolled by Chinese ships and exclusively fished by Chinese fishermen. Last year, China stationed an oil rig near the Paracel Islands, claimed by Vietnam. In 2013, China announced an air-defense identification zone in the East China Sea, claiming control of the skies near Japan and South Korea. Last week, it threatened to do the same in the South China Sea. Already, the Chinese navy has warned off a U.S. Navy surveillance plane that was flying over one of its new artificial islands, broadcasting again and again: "This is Chinese navy. You are approaching our military alert zone. Leave immediately."

What is the U.S. response?
The U.S. insists that China has no sovereignty over those waters. "The United States will fly, sail, and operate wherever international law allows," Defense Secretary Ashton Carter said. "We will remain the principal security power in the Asia-Pacific for decades to come." The U.S. has asked China to stop building islands, but since China refuses, the U.S. is concentrating on reassuring its allies that it will protect freedom of navigation. It has defense treaties with Taiwan and the Philippines, and it just lifted a ban on selling weapons to Vietnam. The Pentagon is reportedly considering plans to send ships and aircraft within 12 nautical miles of the artificial islands, but some U.S. military officials fear that would be needlessly provocative. "There aren't any silver bullets to resolving this," said Assistant Defense Secretary David Shear.

Challenging U.S. naval supremacy
The world's fastest-growing economy, China has channeled a good chunk of its economic growth into the military, raising the defense budget by more than 10 percent a year for most of the past 25 years. In its new military doctrine, China announced it is greatly expanding its navy so it can respond to "provocative actions" in the South China Sea. It is bolstering its already substantial submarine fleet, adding subs armed with nuclear missiles that can reach 4,500 miles. By 2020, China is expected to deploy 342 submarines and missile-firing warships, compared with 243 ships and submarines for the U.S. China commissioned its first aircraft carrier in 2012, and says it will finish two more this year. It also has long-range, anti-ship ballistic missiles that could threaten U.S. aircraft carriers. "The maritime battlefield has been broadened," said Col. Wang Jin, "and China's navy needs to react to protect its rights globally."