Congress alone does not have the power to defund... let's say as an example... Libya?
The House of Representatives does have the power to defund anything by simply not appropriating the money for it. But the Senate can stall the House by not voting for that House bill. Obama's Libyan war can be defunded by the House. But the Senate can refuse to pass that House budget (bill) that does not have the Libya funding. Thus we would have an impasse that resembles what's going on today on the federal budget.
But in the 1970's when the House defunded the Vietnam war the Senate offered no opposition to the House action. Indeed, the Senate passed it and it was signed by the president. There was no fight from the Senate then, unlike today, where we see a socialist Democrat Senate leadership opposing a Republican/conservative House. I remember it well because I was around in the 70s (17 years of age) and thought the Democrats' behavior was outright treason -- just like today.