On Aug. 18, at 2:30 p.m. Pacific Daylight Time, the Defense Department conducted a flight test of a conventionally configured ground-launched cruise missile at San Nicolas Island, Calif. DoD photo by Scott Howe

The US has walked away from the Cold War-era INF Treaty with Russia, and that has given the US newfound flexibility to confront China.

New Secretary of Defense Mark Esper told Fox News Wednesday that China is the Pentagon's "number of one priority," one the department is watching "very carefully."

Commenting on the first US flight test of a post-INF missile, the secretary said, "We want to make sure that we, as we need to, have the capability to deter Chinese bad behavior by having our own capability to strike at intermediate-ranges."

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Having walked away from a Cold War-era arms control pact with Russia, the US is free to confront China in new ways. And it appears determined to do exactly that.

The Pentagon's "number one priority" is China, Secretary of Defense Mark Esper told Fox News Wednesday in his first television interview since he took over the Department of Defense.

"China is the number one priority for this department," Esper, who served as the secretary of the Army before becoming the defense secretary, told Fox News national security correspondent Jennifer Griffin.

Asked whether Russia or China was the greater threat, Esper said that "in the long term, C hina is the greater challenge given its economic might, political weight, and its ambitions."

"They studied us in the years since Gulf War. They've studied us and learned about how we employ weapons, our doctrine," he said, adding that China, which the Pentagon is watching "very carefully," is "clearly professionalizing and expanding the capacity and capabilities of the military in order to push the United States out of that theater."

The new defense chief stressed that the US will not be driven out.

The US recently conducted its first flight test of a conventional ground-launched cruise missile, a weapon that would have been prohibited before the collapse of a Cold War-era nuclear-arms agreement earlier this month.

Read more: The US fired off a previously banned missile, the first since the collapse of a Cold War-era nuclear-arms pact with Russia

The test followed US withdrawal from the Intermediate-Range Nuclear Forces (INF) Treaty in response to alleged Russian violations of the 1987 bilateral agreement, which barred either country from developing or fielding intermediate-range ground-launched missiles, systems with ranges of 500 to 5,500 kilometers.

Read more: The US just entered a great-power arms race in a big way - and Russia and China are panicking

When the US formally withdrew from the treaty at the start of August, the secretary of defense explained that the Department of Defense would "fully pursue the development of these ground-launched conventional missiles," calling it a "prudent response" to Russian actions.

But, since then, Esper has been keenly focused on China.

Responding to a question about which country the US was sending a message with its first post-INF Treaty missile test, the defense chief said, "We want to make sure that we, as we need to, have the capability to deter Chinese bad behavior by having our own capability to strike at intermediate-ranges."

During a recent trip to Asia, he suggested that he would like to see the US deploy intermediate-range missiles in the region "sooner rather than later."

China - never a party to this pact - has been developing missiles in this range for decades. "Eighty percent plus of their inventory is intermediate-range systems," Esper told reporters during the trip. "It "shouldn't surprise [China] that we would want to have a like capability."

For example, China's DF-21D mid-range ballistic missile has an estimated range of 1,500 km and has been dubbed a "carrier killer" for its professed ability to strike flattops while the DF-26 is an intermediate-range missile nicknamed the "Guam Express" for its ability to eliminate strategic US bases in the Pacific theater, such as Andersen Air Force Base on Guam.

While some observers are concerned about the possibility the US is triggering an arms race, others argue that this is a sound strategic evolution given the actions taken by Russia and China.

"We want China's leadership to wake up every morning and think 'This is not a good day to pick a fight with the United States or its allies,'" Tom Karako, a missile-defense expert at the Center for Strategic and International Studies, previously told Insider.

Read more: China warns it will not 'sit idly by' while the US moves to put new strike missiles on its 'doorstep'

China has issued multiple warnings to the US on this issue. "China will not just sit idly by and watch our interests being compromised," the Chinese Ministry of Foreign Affairs warned recently. "What's more, we will not allow any country to stir up troubles at our doorstep. We will take all necessary measures to safeguard national security interests."

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