Recent statements from the Republican Party’s presidential hopefuls on foreign policy don’t inspire much confidence in their ability to take over from Barack Obama in 2016.

Property tycoon Donald Trump has proposed to make Mexico pay for a border fence and insists that all America needs to do is “get tough” with its enemies.

Scott Walker, the governor of Wisconsin, has called on the man he hopes to succeed to cancel Chinese president Xi Jinping’s first state visit to the United States next month because his country continues to manipulate its currency, persecute Christians and throw its weight around in the South China Sea. Aren’t those the sort of things the leaders of the two most powerful countries on Earth may want to talk about?

Chris Christie, the governor of New Jersey, went so far as to blame Obama’s high borrowing for the recent stock market slide which the rest of the world attributed to the slowdown in China.

Blaming Democrat “weakness” for foreign policy challenges is standard Republican repertoire. As are promises from presidential candidates to get “tough” on America’s rivals.

Marco Rubio, the Republican senator from Florida, is supposed to be more thoughtful. Although he was only elected in 2010, Rubio has tried to make a name for himself as serious foreign-policy thinker.

The Atlantic Sentinel has raised doubts about Rubio’s grasp of world affairs before. He supported the 2011 intervention in Libya but then turned round on the Obama Administration, insisting that a wholehearted commitment of American forces would have prevented a worse civil war in the North African country. He later took Obama to task for ending half a century of Cuban isolation, characterizing his reversal of such an obviously failed policy as “part of a long record of coddling dictators and tyrants.” And he incredulously claimed that the Democrat didn’t want to defeat Islamic State militants in Iraq and Syria because he was afraid to “upset” Iran — even though Iran is fighting the same jihadists.

A recent contribution of Rubio’s to Foreign Policy magazine is only a little more levelheaded.

The article consists mostly of laments about Obama’s failure to involve America more thoroughly in both the civil war in Syria and the fight against the self-declared Islamic State in that country and Iraq — without really explaining why he should have.

Rubio argues that the Islamic State inspires homegrown terrorism — which is why the United States are leading a coalition of Arab and Western countries in an air war against the group. But Rubio makes no compelling argument for why it should expand that effort. Nor does he so much as hint at a reason for toppling Bashar Assad, Syria’s heinous dictator.

The senator only has two concrete suggestions. One is to impose a no-fly zone over Syria, something the Obama Administration decided against. Another is to work with allies in the region, notably Jordan and Turkey, to erect safe havens in Syria’s border areas “where the moderate opposition can begin to govern free of the threat of regime (or Islamic State) attacks.”

That is wishful thinking. There is virtually no moderate opposition left. Less because — as Republicans insist — the Obama Administration didn’t back it up strongly enough in the beginning of the revolt (although it may have helped) but rather because Assad was determined to crush any credible alternative to his regime and battle fanatical Islamists instead so the world would fear the consequences of removing him from power.

Rubio’s trust in America’s allies may also be overstated. The United States nominally support Turkish efforts to carve out a “safe zone” in northern Syria, the real objective of which is to thwart the very Kurdish militants who have been among the bravest and most effective in keeping the Islamic State at bay.

The choices are simply not as clear-cut as Rubio would have it.

He is right that military successes against the caliphate will only do so much. “We must first have in place a political strategy to mobilize significant Sunni Arab opposition to this terrorist group, both within Syria and Iraq and in the broader region,” he writes.

How would Rubio accomplish that?

He believes that America still has influence in Iraq to impose a power-sharing agreement that would give the country’s predominantly Sunni areas assurances that that “their rights will be respected by Baghdad,” whatever those rights are. But if the United States were unable to enforce such an agreement when it still had troops in Iraq, what makes Rubio think it has enough influence left to do so now?

Sunni mistrust of Iraq’s Shia-dominated central government, which has sidelined them for years, is well-grounded and a structural cause for the Islamic State’s emergence. But there is good reason to believe that once the group starts suffering military setbacks, it will lose whatever governing appeal it has and Iraq’s Sunnis will look for protection elsewhere.

The biggest impediment to putting together a significant Sunni opposition to the Islamic State, according to Rubio, is the administration’s “ill-considered and unreciprocated outreach to Iran.”

Our allies are alarmed that the president plans to grant Iran generous sanctions relief, potentially worth hundreds of billions of dollars, without demanding the dismantlement of its nuclear program.

Although much of Iran’s nuclear program actually would be dismantled under the international agreement that was reached in July, Rubio is right that many Arabs are fretting about what it will mean for future power relations in the Middle East. If America manages to improve its relationship with Iran, it could reduce its reliance on the authoritarian Arab regimes in the region to balance against the Shia state — which would mean fewer weapons contracts and income for them.

But the Arabs’ interests aren’t America’s. Neither wants to see Iran dominate the region and imperil the free flow of oil to more developed parts of the world. But America also has interests in common with Iran. Both are threatened by radical Salafism — the very ideology peddled by some of America’s “friends”. Both oppose the Islamic State as well as the Taliban in Afghanistan. So does India which seeks to strengthen its security ties with the United States in order to balance against China’s rise in Asia but which is also dependent on Iranian oil and gas supplies. American-Iranian discord stands in the way of both a great power settlement in Afghanistan and closer American-Indian cooperation.

Iran is not America’s friend and should not be trusted. But it cannot be isolated in perpetuity either. Republicans like Rubio seem oblivious to the risks: Iran could seek an alliance with a now equally aggrieved Russia, raising the specter of Iranian-Russian domination in the Caspian Sea region and Russian naval access to the Persian Gulf and Arabian Sea, a longtime Russian strategic goal that America successfully blocked during the Cold War.

For America, it is far more important that India joins its liberal, maritime world order and helps prevent China from establishing a “new type of major power relations” that is based on intimidation and fear than it is to keep the Arabs happy.

It is even more important to stave off any possibility of a Sino-Russo-Iranian condominium in Central Asia that would represent the sum of all American fears: a single, hostile entity dominating the bulk of the Eurasian landmass.

There are plenty of conservative commentators and strategists in the United States who understand this. There are thoughtful Republican politicians as well — just not, it seems, very many at the presidential primary level.

If someone like Rubio must be considered a serious thinker on foreign policy, it is little wonder that Americans are steadily losing their trust in Republicans to manage the country’s foreign affairs.