Spend some time staring at the famous painting "Washington Crossing the Delaware," and you can't miss the ice. It's everywhere. Cold weather became part of Washington's military strategy, says Alex Robb, an educator at Washington Crossing Historic Park outside Philadelphia. "It does a lot to impede the crossing and endanger the whole operation," he said, "but it actually becomes our shield." At the end of 1776, after a string of losses, Washington's army was on the verge of collapse. But Robb