Challenges of the 21st Century: USA

In President Clinton's 1999 State of the Union address he looked ahead to the turn of the millennium and gave the American people an important message. He said, "My fellow Americans, this is our moment. Let us lift our eyes as one nation, and from the mountaintop of this American Century, look ahead to the next one." A new century was ahead that promised unknown challenges and an uncertain future. These included a contested election, hurricane Katrina, financial meltdown, war, terrorism, etc. Which of the challenges of the 21st century has proved most significant and why? What challenge will have the greatest impact on the future?