(CNN) President Donald Trump took America and the world on a wild ride last year.

His norm-shattering first year in office never failed to shock, and through his love of confrontation, insatiable demands for attention and deliberate clawing at political, cultural and racial fault lines, Trump has delivered an era of unpredictability and raw nerves that is sure to intensify in 2018.

The new year brings deepening crises, confrontations and events that could shake Washington to its core and trigger shock waves that will test American unity, global peace and the cohesion of the Trump presidency itself.

Special counsel Robert Mueller's Russia investigation could come to fruition in 2018, and any finding of wrongdoing by Trump or close associates could cause a constitutional imbroglio.

Abroad, the North Korean crisis -- a brewing showdown for 65 years across the world's last Cold War frontier -- is reviving fears of nuclear war for the first time since the demise of the Soviet Union.

And one particular date looms on the calendar: November 6, when congressional elections take place that could see a Democratic wave neuter the Trump presidency. But if Trump -- currently with an approval rating below 40% -- can beat the midterm curse, possibly lifted by a rising economy, he could position himself for re-election in 2020.

Then there are the national and international crises -- mass shootings, economic meltdowns, terror attacks, natural disasters and geopolitical confrontations -- that test every White House and shape each year's politics.

The President, cheered by his tax reform victory and sated by days of holiday golf at his Florida resorts, cannot wait to get back into the fray.

"We're going to have a great 2018 . It's going to be something very, very special. It's all kicking in," Trump told guests at his lavish New Year's Eve ball at Mar-a-Lago in the dying moments of 2017.

"We have some pretty good enemies out there, but step by step they're being defeated," he added. "Everybody's going to love what's happening in our country, because we're taking this big beautiful ship, and we're slowly turning it around."

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Waiting for Mueller's next move

Four Trump associates have been been charged and after reaching a plea deal with former national security adviser Mike Flynn , Mueller may be taking aim at the President's inner circle.

Trump supporters and the conservative media have whipped up a fierce effort to discredit Mueller. In the meantime, the special counsel's silence deepens the mystery over how and when the probe will end.

There are so many questions: Will Mueller deliver a report alleging collusion with Russian election meddling in the 2016 campaign or even recommend the opening of impeachment proceedings? Will Trump seek to pardon former aides and any family members who might be implicated?

Is Mueller preparing more blockbuster indictments? Will political tensions tear apart congressional probes into Russia's election operation? And will congressional Republicans, who celebrated Trump in an extraordinary victory party after the tax win, hold him accountable if necessary?

Trump's lawyers have told CNN that the President is much less nervous about the Mueller probe now, and they expect it to wrap up soon and exonerate him.

But top Democratic Sen. Richard Blumenthal of Connecticut predicted Mueller is just getting started.

"Trump denial of Russian collusion rotten at core and doomed to unravel," Blumenthal tweeted on Saturday. "Expect more serious convictions and indictments early in 2018 as Special Counsel climbs ladder of criminal culpability - and more panicky, preemptive attacks from Republicans."

They can't both be right.

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Nuclear brinksmanship with North Korea

In an alarming New Year's message, the isolated state's leader, Kim Jong Un, warned that the entire United States was within range of his missiles and the nuclear button "is always on the desk in my office."

While that may be an exaggeration of North Korea's capabilities, 2018 could be the year Pyongyang succeeds in putting a nuclear tip on an intercontinental ballistic missile, leaving Trump with the agonizing choice of whether to accept the new status quo or launch military action that some experts fear could degenerate into the bloodiest conflict since World War II.

Adm. Mike Mullen, former chairman of the Joint Chiefs of Staff, warned Sunday on ABC's "This Week" that the US was now "closer to a nuclear war with North Korea than ever."

The US position that Pyongyang must disarm, and the North Korean one that it will not stop until it can demonstrably hit the US, leaves no obvious off ramp. But Kim's recent comment that he wanted to ease border tensions with South Korea ahead of February's Winter Olympics may offer hope for dialogue.

China will continue its rise to challenge US global dominance. US-Russia relations, apart from the personal warmth between Trump and Russian President Vladimir Putin, are deteriorating quickly.

Trump's tough new Middle East policy, targeting Iran's regional power plays, is sending tensions soaring. US relations with traditional allies in Europe have been frayed in the age of "America First," and the year's first humanitarian crisis is primed to erupt in Yemen, as an Iran-Saudi Arabia proxy war rages.

Trump's congressional majorities at stake

Democrats, fired up by strong performances in off-year elections in Virginia and New Jersey and by capturing an Alabama US Senate seat, sense a chance to recapture the House and the Senate in an election that will inevitably be a referendum on the turbulent Trump administration.

Trump is already itching to be involved, though an approval rating in the mid-30% range may make him an unwelcome guest for vulnerable Republicans.

Early in the year, GOP primary duels could escalate the raging Republican civil war between anti-establishment firebrand Steve Bannon and party leaders, who warn that his insurgent candidates could cost them power in Washington.

Republicans will hope that a racing economy, low unemployment and a booming stock market will lift Trump's popularity. The map may also count against Democrats, as they must defend 10 Senate seats in states Trump won in 2016, and redistricting has made many Republican House seats more safe.

CNN poll last month gave Democrats an 18% advantage in a generic ballot against Republicans.

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How will Republicans respond?

Republicans are already facing a logjam in Congress, under pressure to fund the government, renew a children's health insurance program and solve the conundrum over the status of thousands of undocumented migrants brought to the US as children whose legal status will expire within months.

Those issues will test the new spirit of unity between Trump and congressional Republicans forged by the tax triumph.

Already, GOP leaders are at odds over what to do next. House Speaker Paul Ryan wants to pursue welfare and entitlement reform. Senate Majority Leader Mitch McConnell warns that his now one-seat majority might dash such high hopes, and the President is pushing a trillion-dollar infrastructure plan that would need Democratic votes.