President Trump confronts complicated problems as the investigation widens into Russia’s attack on our political system. But his responsibilities are simple: A month ago, he swore an oath that he would “faithfully execute” his office and “preserve, protect and defend the Constitution of the United States.”

That’s apparently easier said than done. In a rambling news conference Thursday and his blizzard of tweets, Trump has dismissed inquiries into his campaign’s contacts with Russia and denounced leakers as “low-life” and “un-American.” These statements seem more likely to confound ongoing investigations than faithfully execute his role as chief executive.

Michael Flynn’s forced resignation as national security adviser this week, after he concealed details of his contacts with a Russian diplomat, has been blurred by Trump’s contradictory comments. So it’s worth going back to basics: Why was the United States expelling Russian spies at the time Flynn made his late-December call to Russian Ambassador Sergey Kislyak? Why would Flynn have hidden for weeks that he talked with Kislyak about those anti-Russian sanctions, or have denied it to the FBI, as The Post reported late Thursday? What would Trump have known about these issues?

You don’t need leaks of classified information to understand why Flynn’s dealings with an aggressive Russia were inappropriate. You just need to look at the public record.

The seriousness of Russia’s assault on America first became clear on Oct. 7, when the intelligence community released a statement charging that “Russia’s senior-most officials” (meaning President Vladimir Putin) had launched a cyberattack “intended to interfere with the U.S. election process.”

Intelligence officials had been briefing members of Congress about the Russian activities since the summer. Adam B. Schiff (Calif.), the ranking Democrat on the House Intelligence Committee, had pushed the White House since September to respond, to no avail. FBI Director James B. Comey, meanwhile, had decided against disclosing the bureau’s own preelection investigation of possible links between Russia and the Trump campaign.

So on Election Day, the public wasn’t aware of the growing belief among intelligence analysts that Russian hackers were trying to help Trump and hurt his rival, Hillary Clinton. That judgment was shared many weeks after the election, in a Jan. 6 report that said Russia sought to “denigrate Secretary Clinton and harm her electability and potential presidency” and that the Kremlin “developed a clear preference” for Trump.

President Barack Obama finally took decisive action on Dec. 29, when he announced sanctions including expulsion of 35 Russian intelligence operatives and closure of two “vacation” compounds, on New York’s Long Island and Maryland’s Eastern Shore, which the Russians were using to collect signals intelligence. (The loss of those monitoring platforms may explain why a Russian spy ship carrying a forest of antennae sailed up the Delaware coast toward Connecticut this week.)

Here’s where the timeline gets intriguing: The White House said in a Dec. 29 conference call with reporters that Obama had informed Trump about the impending sanctions on Dec. 28, one of the days when Flynn communicated with Kislyak, according to the Trump team. (U.S. officials told me a call took place Dec. 29.) Hours after the expulsion was announced, Trump issued a bland statement: “It’s time for our country to move onto bigger and better things.”

We now know that Flynn promised Kislyak that Trump would “review” the U.S. reprisals — a fact Flynn withheld from Vice President Pence and the public for weeks. Flynn finally shared his version with the conservative Daily Caller on Monday, the day he was fired. He said his conversation with Kislyak “was about the 35 guys who were thrown out. . . . It was basically, ‘Look, I know this happened. We’ll review everything.’ ”

Flynn’s promise to review the case evidently encouraged Putin to forgo the usual tit-for-tat retaliation, despite an initial Kremlin statement that there was “no alternative to reciprocal measures.” On Dec. 30, Putin said that rather than taking immediate countermeasures, he would instead seek “to restore Russian-U.S. relations based on the policies of the Trump administration.”

Trump tweeted later that day: “Great move on delay (by V. Putin) — always knew he was very smart!”

Given the magnitude of Russia’s cyberattack on the United States, it remains puzzling that Flynn and Trump were so cavalier about the U.S. government’s attempt to hold Moscow accountable. That’s one reason investigators keep asking what contacts the Trump team had with Russia before the election. Trump said Thursday there hadn’t been any. Yet Russian Deputy Foreign Minister Sergei Ryabkov said Nov. 10: “Obviously, we know most of the people from [Trump’s] entourage.”

The FBI and the Senate Intelligence Committee are investigating the scope of Russia’s pro-Trump activities. Inevitably, there will be leaks, but that issue is a red herring. For all Trump’s talk about “fake news,” the country needs answers.

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