Anthony Zurcher
She's in it to win it… again.
For months, if not years, it was a question of when, not if, Mrs Clinton would announce she is making a second bid for the US presidency. Now that question has been answered.
With the parlour guessing games over, Mrs Clinton faces the long, gruelling road that the US political system demands of its would-be presidential aspirants. The Iowa fairground flesh-pressing, the blustery New Hampshire doorstepping, the nonstop cross-country flights and seemingly interchangeable stump speeches, public rallies and fundraising events that drag on for month after endurance-ebbing month.
All of this takes place under the eternal scrutiny of a press corps hungry for a story and political opponents looking for the choice soundbite or misstep that can sink a candidacy.
This, of course, won't come as any surprise to the former secretary of state. She went through two presidential campaigns with her husband in the 1990s and her own bid - which came up just short to Barack Obama - in 2008.
This time around, however, Mrs Clinton's path to the Democratic nomination appears much easier. Unlike 2008 there's no inspirational, once-in-a-generation opponent like Mr Obama waiting in the wings. There's not even a charismatic, battle-tested candidate like former vice presidential nominee John Edwards or a Hispanic governor with foreign policy chops like Bill Richardson in the field.
But if Mrs Clinton's nomination campaign will be easier, actually winning the presidency could be just as difficult - or more so. Unlike 2008, the Democratic nominee will be defending eight years of her party's rule, with all the baggage that comes with it. Instead of facing a Republican Party on its heels, fresh from massive losses in both chambers of Congress, a nominated Mrs Clinton will have to defeat a Republican candidate with the political wind at his back.
Mrs Clinton also faces an ongoing congressional investigation into her response to the attacks on the US consulate in Benghazi, Libya - which has already spun into controversy over her privately managed email server while secretary of state.
If this is Hillary's moment, she's going to have to earn it. And the mistakes that sunk her 2008 campaign against Mr Obama - if they aren't addressed - could prove equally fatal to her 2016 presidential efforts.
Mrs Clinton's first campaign was beset by bickering among her advisers, for instance. She replaced her campaign manager midway through the primary calendar, and every week it seemed there was a new anonymous leak, as various factions within her camp vied for power.
Mrs Clinton was also criticised for running a campaign that showed little personality - controlled to the point of appearing programmed. It's why the moments she did reveal a more human side, such as when she choked back tears in New Hampshire or touted the "18 million cracks in the glass ceiling" after conceding defeat - were so memorable.
A lack of a unifying vision driving her candidacy has also been identified as a key flaw in her 2008 bid. She seemed content to run on her resume and ability to be "ready on day one to solve our problems". Such a nuts-and-bolts approach melted in the face of Mr Obama's appeal to transcending partisanship and "hope and change" rhetoric.  So will this time around be different? Mrs Clinton has put together a more diverse team of advisers - including some of the very Obama campaign veterans who beat her eight years ago. Early indications are she will attempt to soften her image by emphasising her new role as a grandmother.
According to the Washington Post, she plans to launch a "go-slow, go-small" strategy of appearing in more personal venues to highlight her "humour, humility and policy expertise".
As for a reason why she's running … that remains to be seen. There have been hints that she will embrace the economic populist wealth-gap message most notably touted by Massachusetts Senator Elizabeth Warren. She could also emphasise women's rights, both in the US and internationally - a subject that has been close to her heart for her entire career.
"Little by little, Mrs. Clinton is taking steps that suggest she has learned from the mistakes, both tactical and personal, of her failed candidacy," writes Amy Chozick of the New York Times.
Republican uber-strategist Karl Rove, not surprisingly, isn't buying it.
"She is someone searching for a rationale to run, rather than seeking how best to present it to the public," he writes in the Wall Street Journal. "Slogans and soft lighting can't substitute for real convictions and an authentic sense of purpose."
Mrs Clinton's candidacy has a lot going for it, of course. The simple fact that she's a veteran of national campaigns - and has faced the harsh spotlight for decades - is a strength that puts her ahead of most opponents, both Republican and Democrat.
She also won't want for money. Mrs Clinton, writes the Daily Beast's David Freedlander, is about to "embark on a fundraising push that the Clinton camp says will dwarf anything seen in the history of presidential politics" - topping the $1bn spent to re-elect Mr Obama. There are also the built-in demographic advantages revealed by two straight Democratic presidential victories. The electorate that comes out in presidential years is increasingly diverse and youthful - and leans to the left.
Mrs Clinton - and Democrats in general - had best hope this trend continues and that Mrs Clinton is ready to run the race of her life.
For Hillary Clinton all this is one more - and likely last - chance to realise her lifelong personal ambition. But for Democrats - and left-leaning Americans in general - there's so much more at play. Given the paucity of the field of Democrats, and the current alignment of political power in Washington, DC, Hillary Clinton is the one - the only - hope for cementing the policy gains made over the last eight years.
"Her campaign has become a single point of failure for Democratic politics," writes the New Republic's Brian Beutler. "If she wins in 2016, she won't ride into office with big congressional supermajorities poised to pass progressive legislation. But if she loses, it will be absolutely devastating for liberalism."
If she loses, Republicans in Congress and the executive agencies will have free reign to advance their policy agenda.
Healthcare and financial reform passed under Mr Obama will be rolled back. Aging liberal Supreme Court justices could be replaced with young conservatives. Environmental regulations enacted through presidential action will almost certainly be lifted.
For Mrs Clinton the stakes are high. For Democrats, they couldn't be higher.–BBC

2016 runners and riders
Only Cruz and Paul have formally declared but these are some of the names to watch:
*     early Republican frontrunner is Jeb Bush
*    but New Jersey Governor Chris Christie could battle Bush for the party's centre ground
*    darling of the Tea Party is Texas Senator Ted Cruz
*    firebrand liberal Elizabeth Warren is championed by many in the Democratic Party
*    libertarian Rand Paul has his supporters - and enemies - among Republicans
*     Hillary Clinton will have learnt much from her failed campaign of 2008