A fusion of two phrases:

• "we'll cross that bridge when we come to it" -- we'll deal with problems as they arise. Not always a good policy - if your name is Hannibal and you're travelling with elephants, for example;
• "burn your bridges behind you" - making irrevocable decisions. Retreating troops will burn the bridges behind them to slow up an advancing enemy. If you do this to advancing troops, you've committed them irrevocably to the assault. See burn one's bridges.