By any measure, 2015 was a remarkable year in the world of politics. Here's a look back at some of the most compelling stories of the past 12 months:
The pope storms DC
In September, Pope Francis captivated Washington when he became the first pontiff to visit Capitol Hill.
Those themes were central to his 50-minute speech before a packed joint session of Congress.
"You are called to defend and preserve the dignity of your fellow citizens in the tireless and demanding pursuit of the common good," Francis said, "for this is the chief aim of all politics."
For then-Speaker John BoehnerJohn BoehnerPaul Ryan sells out conservatives with healthcare surrender Matt Schlapp: 5 lessons Trump, Ryan must learn from healthcare debate Nunes rebuffs calls for recusal MORE, the visit was a victory years in the making.
The Ohio Republican, a devout Catholic, had invited popes to Capitol Hill since he first came to Congress more than two decades ago. Francis was the first to accept.
"What a day," Boehner said afterward. "What a moment for our country."
It would prove to be one of his last acts in Congress.
On the morning after the pope's address, Boehner shocked Capitol Hill by announcing that, after 25 years in Washington, he would resign rather than do battle with the restive conservatives in his party.
"I don't want my members to have to go through this," Boehner said. "I certainly don't want the institution to go through this."
The move exposed a dearth of depth on the Republicans' leadership bench. Majority Leader Kevin McCarthy (R-Calif.), the early favorite to take the gavel, was quickly forced to abandon his bid under threat from the same conservatives who ousted Boehner.
Those dynamics ultimately led Rep. Paul RyanPaul RyanRyan says key ObamaCare payments will continue during House lawsuit The Hill's 12:30 Report Rove: Freedom Caucus did Trump ‘grave disservice’ MORE (R-Wis.) to step reluctantly into the spot. Ryan, then the Ways and Means Committee chairman, insisted he didn't want the job. But without another candidate able to unite the party’s factions, he had little choice.
It remains to be seen how Ryan manages his right flank in 2016, when he'll have to tackle tough issues of government spending ahead of November's elections.
Obama rejects Keystone
The fight over the fate of the Keystone XL pipeline — an enormous project slated to deliver oil sands crude from Canada to U.S. refineries on the Gulf Coast — had put President Obama in a box for years.
Among the project’s loudest proponents were labor unions, central to Obama's and the Democratic base. But the project was anathema to another important bloc: liberal environmentalists.
Obama's initial response was to delay the decision, and many wondered if he wouldn't simply kick the verdict to the next administration.
But in November, he instead rejected the pipeline outright with the argument that it "would not make a meaningful, long-term contribution to our economy."
In the same speech, he endorsed the international climate negotiations that were soon to begin in Paris.
“We want to prevent the worst effects of climate change, and the time to act is now,” he said.
The debate may not be over. Republicans could still try to overturn Obama's decision, meaning the issue may fall to the next president after all.
The debate over gun control had all but fizzled on Capitol Hill after the Senate in 2013 quashed legislation expanding background checks in the wake of the shooting massacre at Connecticut's Sandy Hook Elementary School.
Then came 2015, and a string of high-profile mass shootings stretching from Charleston, S.C., where nine people were killed at a historic black church, to Roseburg, Ore., where a 26-year-old gunman killed nine people and wounded nine others before fatally shooting himself.
Every case brought new pleas from Obama for Congress to strengthen the nation's gun laws.
"We have a pattern now of mass shootings in this country that has no parallel anywhere else in the world," he said following the December massacre in San Bernardino, Calif., in which 14 people were killed.
Congressional Democrats pushed to attach several gun control measures to the year-end government funding bill, including a proposal to bar people on the FBI's no-fly list from buying guns and another to end a federal ban on gun violence research.
Those efforts went nowhere in the Republican-controlled Congress.
Race and criminal justice
America's long-running, tense discussion about race and bias in the criminal justice system exploded in 2015 after a string of highly publicized killings of unarmed black men at the hands of the police.
The tragedies sparked street protests in numerous cities, focused new attention on race as a factor in law enforcement, amplified calls for tougher laws to combat race-based abuses such as profiling, and renewed the debate about the role of the federal government in overseeing local law enforcement agencies.
The headlines also gave weight to the Black Lives Matter movement, which went after politicians of all stripes — notably Democratic presidential hopefuls Hillary Clinton and Bernie Sanders — with charges that both parties have neglected to prioritize the fight for reform.
The issue has been a challenge for Obama, the nation's first black president, who has faced accusations that he has done too little to seek systemic improvements. The president has responded with repeated calls for a criminal justice overhaul that's been stalled in Congress for years.
Iran nuclear deal
World leaders in July finalized a historic agreement to limit Iran's nuclear capacity.
The deal — struck between Iran, the United States and five other global powers, including Russia and China — aims to block Iran from building nuclear weapons in exchange for the removal of long-standing international oil and financial sanctions.
Supporters hailed it as a vital tool in preventing a nuclear-armed Tehran.
Critics, though, assailed the deal with warnings that it would ultimately enable Iran's nuclear capabilities and threaten the United States and its allies, particularly Israel.
Amid the fight, Boehner stirred controversy by inviting Benjamin Netanyahu to address a rare joint session of Congress, a podium the Israeli prime minister used to deliver a scathing rebuke of Obama's approach.
It wasn't enough. When Republicans sought to block the accord, their proposal lacked enough Democratic support to pass the Senate.
The lingering terror threat resurfaced in horrible fashion late this year with a pair of deadly attacks in Paris and San Bernardino, Calif.
The assaults raised new concerns about Western efforts to contain the influence of the Islamic State in Iraq and Syria (ISIS) and sent Congress scrambling to strengthen homeland security measures before its holiday recess. The attacks also sparked an anti-Muslim backlash that extended to the presidential campaign trail, most notably in Donald TrumpDonald TrumpObama should testify before Senate Intelligence Committee Trump should run Twitter like a business The Hill's 12:30 Report MORE's call to temporarily bar Muslims from entering the country.
The massacres also provoked new condemnations of Obama's plan to allow 10,000 Syrian refugees to resettle in the U.S. over the next year. At least one perpetrator of the November Paris attacks, which left 130 people dead, had allegedly entered Europe as a refugee.
Republicans quickly drew up legislation to halt the program, and the bill passed the House with 47 Democrats on board. But facing a sure veto from Obama, it went no further.
Gay marriage legalized
The Supreme Court in June delivered a huge victory for gay and lesbian couples when it ruled they have a constitutional right to get married.
The decision dissolved the patchwork of often conflicting state laws and ignited a firestorm o about states’ rights, religious freedom and the reach of the court.
The 5-4 ruling swung on the vote of Justice Anthony Kennedy.
"It is demeaning to lock same-sex couples out of a central institution of the nation’s society, for they too may aspire to the transcendent purposes of marriage," Kennedy wrote.
The post-ruling transition has not always gone smoothly. Kim Davis, a county clerk in Kentucky, drew national headlines over the summer when she refused to issue marriage licenses to gay couples, citing her Christian beliefs.
Davis spent five nights in jail but won a victory this month when newly elected Gov. Matt Bevin (R) issued an order removing the names of county clerks from state marriage licenses.
The budget fights that have practically defined Capitol Hill in recent years persisted through 2015.
After winning Senate control last year, Republicans led both chambers of Congress for the first time in a decade. But Obama's veto pen, combined with the Senate filibuster and discord in the GOP ranks, made it inevitable that tough negotiations lay ahead.
The conflict, by design, started early, as Republicans in 2014 had crafted a year-end spending package that funded the Department Homeland Security (DHS) only through February, a strategy designed to appease conservatives who wanted to defund Obama's executive action halting some deportations.
But, facing a DHS shutdown, Boehner surrendered to Obama's demand for a bill with no amendments, angering conservatives.
Similar dynamics governed September's debate over a stopgap bill to prevent a full government shutdown, with conservatives this time demanding provisions to defund Planned Parenthood. Boehner bucked them again, losing 151 Republican votes in the process.
The process ran more smoothly under newly elected Speaker Ryan, who this month succeeded in passing a yearlong spending bill and a sweeping tax package. But that relative tranquility may not last.
Ryan has vowed a bottom-up approach next year that lends more power to his troops. Whether he can meet that promise and negotiate spending deals Obama will sign remains an open question.
Trump shocked the political world this year, not just for quickly vaulting to the top of the Republican presidential field but also for his staying power.
The billionaire business mogul's campaign has been controversial from the start — he launched his bid in June with a fiery speech characterizing most Mexican immigrants as criminals — and has grown even more contentious since then.
His running confrontation with Megyn Kelly of Fox News has drawn howls from women's groups, who accuse him of misogyny; his calling for a ban on Muslims entering the U.S. also sparked outrage in some quarters; and he more recently drew condemnation for taking a vulgar shot at Clinton.
But his unscripted, no-apologies approach has resonated with voters resentful of political insiders. Trump has dominated more establishment-friendly GOP candidates such as former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Sen. Marco Rubio (Fla.).
Many experts still expect Trump to fade when the primary polls open. But no one denies he's tapped a vein of widespread voter discontent — a message no candidate in either party has been allowed to ignore.