Secretary of Defense Jim Mattis, right, greets John Bolton, President Trump’s choice to be national security adviser, as he arrives at the Pentagon on March 29. (Reuters)

John Bolton takes over as national security adviser today in the midst of a new national security crisis: another chemical attack in Syria. President Trump responded with strongly worded tweets blaming both Russian President Vladimir Putin and Syrian President Bashar al-Assad, warning there would be a “big price to pay.”

Bolton is known for his hawkish views and general belief that the United States should use military power preemptively. Trump’s views on conflicts like that in Syria have been inconsistent, even contradictory. As recently as last week, the president was apparently disagreeing with his own Pentagon when he said the United States should get out of Syria immediately. Yet one of his tweets during the weekend implied that stronger U.S. action might have stopped Assad.

But that’s only the latest crisis. Bolton and the Trump administration will also be confronting the looming diplomatic encounter with North Korea and ongoing challenges like the war in Afghanistan and the fate of the Iran nuclear deal. As they do so, keep this basic point in mind: In many of these regional conflicts, the United States has a limited amount of leverage or is at the mercy of forces beyond its control.

That doesn’t mean the United States has no power, or that individuals like Bolton and Trump don’t matter; my book about how presidents shape military interventions argues that they do. But international relations scholars know well that international forces and other countries’ interests and domestic politics can be powerful constraints on what the United States can do to shape events, particularly in regional conflicts. That’s important to remember in the constant ebb and flow of news.

Several recent TMC posts examine those constraints in different parts of the world. Let’s take a look.

China will have a big say in what happens next with North Korea

After Trump’s announcement that he would meet with North Korean leader Kim Jong Un, China and North Korea had a surprise of their own to unleash: Kim made a secretive visit to Beijing. Here at TMC, Rush Doshi explored three takeaways from the meeting. Doshi concludes, “Beijing has strong incentives to push for full involvement in the forthcoming talks. Diplomacy over North Korean denuclearization remains fluid and volatile, but one thing remains clear: Xi Jinping won’t be content to stay on the sidelines.”

China itself has a complex and volatile relationship with North Korea. But whatever Bolton’s and ultimately Trump’s plans for North Korea, they must account for China’s increasingly active role.

Regional conflicts have their own internal dynamics that resist U.S. intervention

Recently, there has been renewed talk of a peace settlement in Afghanistan, where much to Trump’s apparent chagrin, U.S. forces remain on the ground. But how likely is a peace deal?

As Asfandyar Mir lays out clearly in his recent Monkey Cage post, internal dynamics and domestic politics within Afghanistan could derail peace at any step: during negotiations, defining exactly what a deal would look like, dealing with spoilers, putting a deal into force and, finally, possibly spilling back into civil war. The United States has little power to change most of these factors. Indeed, as Greg Jaffe recently wrote at The Post, top military officials now talk about “winning” in conflicts like Afghanistan not in terms of outright victory, but in keeping a permanent presence, an “infinite war.”

Many of Mir’s points about Afghanistan probably also apply to Syria, where realistic military options are hard to identify. Last year, after another alleged chemical attack, the United States struck Syria with a cruise missile. Marc Lynch wrote here that the strike alone wouldn’t shift the overall strategic stalemate in Syria. There is little reason to imagine things are much different this time.

Bolton’s hawkish views come from the era of unchallenged U.S. power after the Cold War — which is coming to an end

Bolton is best known for serving in the George W. Bush administration, when U.S. military power was largely unchallenged. As Joshua Shifrinson argued here at the Monkey Cage, “In an age of overwhelming U.S. military dominance, there is no other country to push back when the United States gets aggressive. As a result, militant and assertive views have more scope to shape policy.” Bolton’s hawkish views are partly a symptom of post-Cold War U.S. world dominance.

Shifrinson concludes that other countries like China may be challenging U.S. economic power, but that the United States still has a clear military advantage. That might make military force look more attractive to many in the White House, “because that is where the United States outcompetes the rest of the world.” Thus, “with U.S. power dominant but waning, Bolton’s elevation reminds us there are few limits to — and some momentum behind — a militaristic U.S. foreign policy.”

Alliances and diplomacy remain important for addressing regional conflicts

Trump and Bolton both appear to disdain alliances and many forms of diplomacy. But the ongoing challenges facing the United States show how important these tools can be.

For example, Stephan Haggard explained here how sanctions have actually worked to bring the confrontation with North Korea to this point. Multilateral coordination, always difficult with sanctions, turned out to be crucial. As Haggard notes, “The concerns now rest with a U.S. administration prone to grand gestures but with a foreign policy bench depleted to some extent by turnover. The United States faces the challenge of coordinating at the multilateral level and with China to maintain the sanctions pressure that will lead to talks.”

Although David Bosco recently argued here that Bolton has an underappreciated record of multilateralism, neither Bolton nor Trump is known for the kind of repeated, patient diplomacy Haggard suggests will be crucial for successful North Korea talks. Indeed, as Mira Rapp-Hooper has pointed out in several pieces here, Trump’s threats against North Korea have more often harmed U.S. alliances than changed the dynamics with Kim.

Taken together, these arguments have a sobering message for the president and his new national security adviser. Trump and Bolton will certainly put their own stamp on U.S. policy over the next few months. But they’ll have to contend with a host of complicated forces beyond their control, and will have less power to shape other regions’ conflicts than they may wish. The United States has had to relearn this lesson many times — often painfully, especially in the wars in Vietnam, Afghanistan and Iraq.