The airship was the very last thing Charles and David Koch expected when they arranged for 200 of their most wealthy and influential friends to spend the weekend in the desert east of Los angeles.  Sponsored by Greenpeace, it hovered over the luxury spa where they had gathered. On its side were pictures of the billionaire brothers along with a words "dirty money".  Then there was the angry mob. around a 1,000 liberal activists spent Saturday and Sunday outside the gates of the Rancho Mirage near Palm Springs. Most waved placards condemning "corporate greed" and "crimes against the environment". By close of play, 25 had been arrested, for trespassing.  It was a circus all right. But a fascinating and quite possibly game-changing one: the trade unionists, environmentalists and assorted lefties assembled outside the resort were staging the first major public protest against the Koch Brothers. They are two of the most influential men who, until now, you may very well have never heard of.  The secretive brothers, aged 75 and 70, have built a fortune of around $35billion (?22bn) through their firm Koch Industries, which has oil, timber, chemical and other energy interests and is the second-largest privately-held company in the United States. Much of that money is quietly spent supporting political advocacy groups which advance what critics call a radical right-wing agenda.  In recent years, the Koch Brothers have given tens of millions of dollars to Republican candidates. Millions more of their dollars have been given to think tanks such as the Cato Institute and Heritage Foundation, and lobbying organisations like the US Chamber of Commerce. They have also helped bankroll dozens of cleverly-named pressure groups, including americans for Prosperity and the Institute for Justice.  Most organisations which benefit from the Kochs' largesse have one thing in common: they help advance an unflinching brand of libertarian conservatism. Some lobby against environmental regulation, or seek to undermine public perception of the threat of climate change, others battle taxes, trade unions and Barack Obama's healthcare reforms.  Many play a crucial role in organising the Tea Party, the headline-prone right-wing movement which likes to tout its "grass-roots" credentials.  Until recently, the Brothers have operated largely in the shadows. But now questions have begun to be asked about their growing role in public affairs. Before November's mid-term elections, the New Yorker ran a lengthy investigative article detailing their "war on Obama". It quoted Greenpeace calling them the "kingpin of climate science denial" and described the ideological network they preside over as the "Kochtopus."  Ever since, public interest in Charles and David Koch's affairs has been quietly brewing; and when the New York Times revealed in October that they had booked the entire Rancho Mirage resort for one of their twice-yearly gatherings of the wealthy and the influential, the seeds of the weekend's protest were sown.  a leaked invitation to the retreat, which finished yesterday, informed guests that they were meeting "to review strategies for combating the multitude of public policies that threaten to destroy america as we know it". These apocalyptic threats included "climate change alarmism and the move to socialised health care", as well as "the regulatory assault on energy". In his covering note on the invitation Charles asked: "If not us, who? If not now, when?"  as news of the event spread, a loose collective of leftist groups - including Greenpeace, several trade unions and citizen's lobby group Common Cause - seized the opportunity to stage a public protest at what it sees as the malign influence of the Koch Brothers and their wealthy friends, who the