Not since North Korea conducted a nuclear test in October 2006 has the decibel level on the Korean Peninsula been so high.

In the past few weeks, Pyongyang’s state-run media have branded South Korea’s new president, Lee Myung-bak, a “U.S. sycophant” and a “traitor” and warned the South that the North had the capacity to “reduce it to ashes.” North Korean fighter jets have buzzed the demilitarized zone, and Pyongyang has test-fired short-range missiles off its western coast.

Many observers have characterized the North’s latest actions as the behavior of an irrational, surrealistically isolated regime. In reality, such moves have always been less a product of paranoia than a calculated way of making political points to the outside world. But to understand what Pyongyang is doing, one must start with what is going on in Seoul.

In late February, Lee, a conservative business leader turned politician, became South Korea’s president. His two more liberal predecessors, Kim Dae-jung and Roh Moo-hyun, had pursued a “sunshine policy” toward North Korea of diplomatic engagement, economic cooperation and humanitarian assistance with no strings attached. Lee proposed a fundamental shift, vowing to link aid and economic collaboration with North Korean “reciprocity” on the nuclear issue.

Lee has proved true to his word. South Korea voted for a U.N. resolution condemning North Korea’s human-rights record. Then South Korea declared it would consider launching a pre-emptive strike on North Korea’s nuclear facilities if they became a military threat.

To the North Koreans, these actions represented a diplomatic betrayal, and the push-back began almost immediately.

North Korea has for years adopted a strategy of tit for tat — responding positively to conciliatory overtures but extremely sharply to pressure.

When the Bush administration sought to pressure Pyongyang to abandon its nuclear program in late 2002 and early 2003, for example, the North reacted by restarting its frozen plutonium reactor. After the U.N. Security Council imposed sanctions following the North’s July 2006 missile tests, Pyongyang defied the international community and tested a nuclear bomb that October.

But when, less than a month later, the Bush administration promised to resolve a dispute over North Korean funds frozen in Macao — addressing long-standing North Korean demands — Pyongyang promptly agreed to return to the six-party talks on the nuclear issue. Barely four months later, a deal to end the nuclear program was concluded.

Pyongyang’s response to the tough talk from Seoul fits into this pattern.

The North-South flare-up comes at a crucial time, with South Korea’s Lee arriving in Washington this week. Meanwhile, Washington and Pyongyang have struggled to implement the February 2007 nuclear deal. The original deadline for disabling facilities and accounting for material was Dec. 31, 2007.

President Bush is eager to avoid renewed confrontation and to secure a denuclearization deal. The hard-line posture of the new South Korean government, however, threatens to undermine this goal.

There’s more than a little irony in this. In the first six years of the Bush administration, Lee’s liberal predecessors pushed for greater engagement with the North, often putting them in conflict with Washington’s more confrontational approach. Now the roles appear reversed.

Thus, much depends on Lee’s meeting with Bush. In a goodwill gesture, the White House has invited Lee to Camp David, a privilege it declined to offer his predecessors despite their repeated requests. It will be up to Lee and Bush to craft a response to the North’s saber-rattling that will keep the nuclear negotiations and North-South rapprochement on track — or face an increasingly dangerous situation on the Korean Peninsula.

(Chinoy, a senior fellow at the Pacific Council on International Policy, is the author of the forthcoming book “Meltdown: The Inside Story of the North Korean Nuclear Crisis.”)