“I have a pen and I have a phone.” That was President Obama’s commitment to take executive action and bypass legislative gridlock between the Democratic-controlled Senate and Republican-dominated House.

The president should take that same pen and draw a red line across the Asia Pacific region in response to China’s threats of force in the Taiwan Strait, the South China Sea, and the East China Sea. The line would also transverse the Korean Peninsula at the 38th Parallel.

Then he needs to pick up that phone and enlist the cooperation of America’s regional allies—Japan, South Korea, Australia, Philippines—as well as friends and security partners like Taiwan, Vietnam, Singapore and Indonesia. He should assure them of Washington’s commitment to maritime and aviation security in the region and seek their material and diplomatic support for that common good.

China’s leaders, and some U.S. commentators, will charge provocation. But such a presidential declaration will ultimately avoid conflict by affirming freedom of navigation and flight as a unifying theme in the regional and international order that has existed for over six decades.

It would not be the first time a U.S. leader drew a line on the map of Asia. Secretary of State Dean Acheson and General Douglas MacArthur, supported by President Harry Truman, did it in early 1950 to declare U.S. strategic resistance to Communist expansion. Unfortunately, their defense perimeter did not include South Korea or Taiwan. Beijing and Pyongyang saw not a red line but a green light, and the Korean War erupted.

To avoid a repeat of such strategic miscalculation, Washington needs to state a clear, coherent deterrent policy covering the entire region rather than responding piecemeal to—or worse, ignoring—individual probes and provocations by China or North Korea.

North Korea’s record of reckless conduct is legion. But China, Pyongyang’s supposedly more responsible senior partner, has been playing its own dangerous game by stirring security concerns in the region. In a series of policy and “legal” declarations, it has asserted ever-widening territorial claims: a “nine-dash line” across the South China Sea, new fishing rules for the region, and an Air Defense Identification Zone (ADIZ) in the East China Sea.

Beijing has done more than issue unilateral declarations of its rights in contravention of the United Nations Convention on the Law of the Sea (UNCLOS) and customary international law. It has also acted aggressively using both civilian and naval craft to interfere with legitimate maritime operations by the Philippines, Vietnam and the United States. Its ships and planes have illegally entered Japan’s waters and airspace and harassed Japanese vessels and aircraft.

In response to China’s growing assertiveness in the region, Washington has announced a rebalancing of military forces to the Asia-Pacific region, but the actual augmentation of naval and air power has been largely symbolic so far.

The U.S. has also sent mixed signals regarding its support for America’s regional allies. Commendably, the Obama administration has pledged to extend the U.S. alliance protection to the Japanese-administered Senkaku/Diaoyu islands. And when China suddenly announced its ADIZ, Washington defied it by flying two B-52s though the zone. But it parted company with its Japanese ally by advising U.S. commercial airlines to honor China’s new rules.

Similarly, the U.S. mediated the resolution of a dispute between China and the Philippines, another ally, over Scarborough Shoal, but did nothing when Beijing violated the agreement and occupied the Shoal.

Washington also needs to discard its policy of “strategic ambiguity” regarding its commitment to defend Taiwan against China’s stated intention to take the island by force if necessary.

If President Obama fails to lay down a convincing red line regarding China’s threatening misbehavior across East Asia, the world may see a disastrous reenactment of the Korean War miscalculation. As Henry Kissinger has written: “We did not expect the attack; China did not expect our response.” Repeating that mutual miscalculation would bring even more tragic consequences for the region and the world.

Joseph Bosco is a senior associate at the Center for Strategic and International Studies. He served as China country desk officer in the Office of the Secretary of Defense and taught a graduate seminar on US-China-Taiwan relations at Georgetown's School of Foreign Service.