NEW DELHI – Stoking tensions with Japan, Vietnam, and the Philippines over islands in the South and East China Seas has not prevented an increasingly assertive China from opening yet another front by staging a military incursion across the disputed, forbidding Himalayan frontier. On the night of April 15, a People’s Liberation Army (PLA) platoon stealthily intruded near the China-India-Pakistan tri-junction, established a camp 19 kilometers (12 miles) inside Indian-controlled territory, and presented India’s government with the potential loss of a strategically vital 750-square-kilometer high-altitude plateau.