W.H. Auden’s famous 1937 poem “Spain” about the civil war there includes this memorable refrain: “Yesterday all the past. . . . But to-day the struggle.”

Today, the struggle for President Biden begins with domestic policy. And it would be nice if Americans could remain inwardly focused for a while, meditating on Biden’s inaugural theme of unity and national reconciliation.

But the world won’t wait. It needs (and mostly wants) a strong United States back in harness. Biden’s task is partly just rebuilding after Donald Trump’s demolition derby. Biden rightly pledged in his inauguration address on Wednesday: “We will repair our alliances and engage with the world once again.”

Restoring support abroad won’t be easy, even with our friends. A poll released this week by the European Council on Foreign Relations registered enthusiasm for Biden’s victory, but 6 in 10 Europeans said they think the U.S. political system is broken, and that China will become more powerful than the United States over the next decade.

Let’s be contrarians for a moment about the repair job that’s ahead: Not everything is broken. U.S. power is partly about continuity and momentum. Sometimes, the generals, diplomats and intelligence chiefs had to defy Trump to maintain sound policy. Still, the simple fact is that not everything the Trump administration did in foreign policy was wrong. Some things are worth sustaining.

At the top of my list of positives is that the United States kept its roughly 2,500 troops in Iraq, which Baghdad wants, despite Iran’s effort to drive them out. Shiite militias backed by Tehran escalated attacks on the U.S. Embassy and other installations — to the point of firing 20 rockets at the embassy compound in December. Further attacks might have given Trump an excuse for launching a bloody reprisal.

The United States deterred Iran and its proxies by using all the tools of government. B-52 bombers flew repeated missions to the Persian Gulf to demonstrate American resolve; the USS Nimitz was turned around at sea and dispatched to the area; top U.S. officials conveyed warnings directly to their Iranian counterparts. The campaign, one State Department official rightly claimed, “was a model of interagency cooperation and of diplomacy, and it worked.”

Now comes the hard part, rebuilding the 2015 nuclear agreement with Iran and adding limits on Tehran’s destabilizing activities in the region. Trump foolishly wrecked that agreement, and he got nothing to show for it except more enriched uranium. Biden should correct that mistake, but carefully.

The Trump administration didn’t capitulate on Afghanistan, either, despite Trump’s fervent desire to bring home all the troops. The United States continued to insist that withdrawal of its remaining 2,500 U.S. troops there must be “conditions-based,” a point that was driven home personally by Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, when he met with the Taliban leadership in December in Qatar.

Here again, Biden will have to decide whether to continue with the conditions-based formula or pull the plug. If he quits Afghanistan without a reduction of Taliban violence, the new president risks making the same mistake he made as vice president in advocating the too-hasty withdrawal from Iraq in 2011. Continuity here would make sense.

A final area where Biden should be careful about jettisoning Trump policy is China. Trump’s harder line on trade and technology controls reflected a broad recalculation of the Sino-American relationship that includes a bipartisan consensus of experts. Trump wasted too much time on squabbles about tariffs. But he was right to argue that the United States should take a more assertive line toward Beijing.

The Chinese are already planning to test the new administration. A new maritime safety law will probably take effect in March, according to a U.S. consulting firm that studies China. To maintain its version of traffic safety at sea, China could prevent foreign ships from entering domestic waters — which are given an aggressively broad definition under the new law.

Trump’s secretary of state Mike Pompeo saddled the new administration with a wave of additional punitive measures against Beijing — some that make sense and some that don’t. Biden’s experienced Asia policy coordinator, Kurt Campbell, will have his hands full. The trick will be to maintain a tough line on trade and technology without starting a new Cold War with China that would harm both sides.

When it comes to foreign policy, Biden will find that everything is new — and everything is the same. Trump’s disruptive approach made a mess, but not every policy stance should be discarded just because it bears his name.

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