U.S. President Barack Obama's recent lifting of the decades-old arms embargo on Vietnam is the latest move in the security game with China, currently centered on the South China Sea. China's intention has long been to be the regional hegemon and Beijing sees physical possession of the South China Sea as necessary to its own security. Control of the various atolls and reefs has been disputed for decades; what has changed is the volume and speed of the island-building campaign.
The most likely trigger for this new phase of offensive expansion was the Obama administration's 2011 announcement of refocusing on the Pacific. Early in the century, with the United States embroiled in conflicts in Iraq and Afghanistan, it appeared that China would have a free hand in Asia. With the pivot, that changed.
Geography matters and China intends to have physical possession of the South China Sea. It has expanded or built up from the sea floor seven islets and reefs in the Spratlys, in addition to having occupied the Paracel Islands, also claimed by Vietnam, and seizing control of Scarborough Shoal, claimed by the Philippines. In 2012, China expanded Sansha City in the Paracels to provide administrative control over its activities in the South China Sea, bolstering its claim that the islands are an integrated part of China. Most importantly, these build-ups allow naval and air patrols, cementing an economic and administrative claim with a military presence.