Strong words and determined action by the United States have backed down a militant China in a disputed area of the South China Sea. Simply put, the United States isn’t prepared to become a paper tiger in response to China’s latest saber rattling.

The message needed to be delivered, particularly considering the Obama administration’s ambiguous record on the defense front.

But concerns remain about China’s further intentions in its so-called Air Defense Identification Zone. Questions about what rules it intends to enforce and how it will do so leave open the possibility of armed clashes with nations that routinely patrol the area, including the United States.

China’s defense ministry recently announced the new ADIZ and published a map showing that a broad area conflicts with a similar set of rules long established by Japan over islands claimed by Japan but contested by China.

U.S. Defense Secretary Chuck Hagel immediately responded, “We view this development as a destabilizing attempt to alter the status quo in the region. This unilateral action increases ... misunderstanding and miscalculations.” A bilateral U.S. defense pact covers any attack on Japanese territory, he said, including the Senkaku Islands that lie within the new Chinese air defense zone.

Secretary of State John Kerry subsequently warned China to “exercise caution and restraint.” Then the United States flew two B-52 bombers through the new Chinese ADIZ and over the islands without filing a flight plan with Chinese authorities. It was later announced that the aircraft were unarmed, but presumably the Chinese were not aware of that fact.

Following the U.S. action, Japanese and South Korean military aircraft and civil airliners also flew through the area without filing flight plans.

China then announced that it had tracked all the flights and added that the new ADIZ wasn’t a “no-fly” zone or an extension of China’s air space, and that it was wrong to assume that the new zone implied that China might attack aircraft entering it without Chinese permission.

But these pacifying words were accompanied by an announced determination to enforce the rules, which state that all aircraft entering the zone must identify themselves, disclose their flight plans and obey instructions from the Chinese military.

China also sent its own military aircraft to patrol the area and track flights.

The apparent intention of China to assert sovereignty over waters and land claimed by Japan and South Korea is an escalation of existing international tensions in the South China Sea and a direct challenge to current international practice.

The United States was right to firmly challenge China. The American response should serve further notice on those nations who have been emboldened by the administration’s timidity elsewhere.

Apparently there really is a red line for the U.S. and its allies in the South China Sea.