Charities and the federal government launched what aid agencies predicted could be the longest and costliest relief effort in U.S. history, as workers began arriving last night in states devastated by Hurricane Katrina, and as the U.S. military organized an intensive response by already stretched National Guard and active-duty forces. The American Red Cross, working in concert with the Federal Emergency Management Agency, called its plan to house and feed tens of thousands of people the biggest response to a single natural disaster in the organization's 124-year history.