The US defense secretary has directed the Pentagon to sharpen its focus on China as Washington marks Beijing its top strategic rival. Lloyd Austin said the initiatives are nested inside the larger US government approach toward China and will help inform the development of Washington’s National Defense Strategy. The details of the directive are classified. The United States routinely sends warships and warplanes to the South China Sea to assert what it calls its right to freedom of navigation, ratcheting up tensions among the regional countries. China has constantly warned the US against its military activities in the region.