As Starboard launches Yahoo proxy fight, Marissa Mayer faces the battle of her life. Again.

Updated at 6:55 a.m. PST with details of Starboard proxy filing.

Just six weeks ago, Yahoo CEO Marissa Mayer appeared to have emerged at least temporarily victorious in her battle to steer the future of her hapless company.

Whatever respite she might have won is proving to be short-lived.

Yesterday, activist hedge fund Starboard announced that it was launching a proxy fight to replace all members of Yahoo’s board of directors, according to the Wall Street Journal. In a letter to Yahoo from Starboard, obtained by the WSJ, the fund argues that “the board and management have continually failed to live up to their own promises, and shouldn’t be trusted with the decision on whether or not Yahoo should remain an independent company.”

The Starboard fund has been advocating for a couple of years now that Yahoo sell its core business to separate its lucrative investment in Alibaba. There seemed to be a growing momentum behind that idea, and there was talk that the company would announce plans to do just that at a board meeting in early February.

Instead, Mayer deftly struck a compromise to buy more time. Even as the board said it would continue to explore strategic options (a.k.a, a sale), it was backing Mayer’s efforts to turn around the good ship Yahoo.

Phew!

But then, just a few days later, Verizon acknowledged that it was considering a bid for Yahoo, serving up a reminder that the guillotine was still poised.

And now, a proxy fight. Pre-Mayer, Yahoo beat back another proxy fight with activist shareholder Carl Icahn in 2008, even as it was fending off an unwanted takeover offer from Microsoft.

Despite the current troubled outlook for Yahoo, Starboard’s campaign won’t be easy. It has until March 26 to nominate its own alternative slate of directors. And getting a majority of shareholders to vote against any incumbent board is always a long shot.

On the other hand, the trends for Yahoo remain bleak. In late February, Yahoo announced it was writing down the rest of the value of Tumblr, an admission that Mayer’s $1-billion acquisition gamble had been a dud.

And this week, eMarketer released a report projecting that Yahoo’s advertising revenues would fall 14 percent this year. Meanwhile, Google is expected to grow ad revenue by 9 percent and Facebook by 31 percent.

Adding to the misery, eMarketer predicts Yahoo’s display advertising business will fall 15 percent and that its search business will drop 12.7 percent.

“As Yahoo trims down its legacy business to focus on its so-called MAVENs, we expect the company to shrink in size relative to its competitors,” said eMarketer senior forecasting analyst Martín Utreras in the report. “A leaner Yahoo, more focused on its core growing segments, will still face stiff competition in an ever more crowded and sophisticated market.”

(Note: MAVENs is really MaVeNS, Yahoo’s word for “mobile, video, native, and social.”)

In any case, Mayer has been underestimated before. But, in the short-term, she’s also not packing a lot of additional ammunition to fend off the barbarians at the gates. The more interesting question may be whether the board has the stomach to fight on.

Do they really believe in Mayer? If not, the new proxy threat makes the prospect of a quick sale a tempting way to put everyone out of their misery.

UPDATED 6:55 a.m. PST: Bloomberg has reported that Starboard has nominated its slate of nine directors to challenge Yahoo. As of yet, there has been no official filing with the U.S. Securities and Exchange Commission, and the names have not been published.

Meanwhile, Fortune has published the full shareholder letter from Starboard to Yahoo investors:

“We believe that Yahoo is deeply undervalued and opportunities exist within the control of management and the Board of Directors (the “Board”) to unlock significant value for the benefit of all shareholders,” the letter states. “Unfortunately, as we have outlined in previous letters, we have been extremely disappointed with Yahoo’s dismal financial performance, poor management execution, egregious compensation and hiring practices, and general lack of accountability and oversight by the Board. We believe the Board clearly lacks the leadership, objectivity, and perspective needed to make decisions that are in the best interests of shareholders.”