Indeed, they will.  Ron Paul's simple argument that "we just walked in; we can just walk out" of our foolish involvements in the Middle East continue to ring true -- and the fact that we can just pull out of Iraq and Afghanistan is well argued by Tom Englehardt at the American Conservative:

We’ve now been at war with, or in, Iraq for almost 20 years, and intermittently at war in Afghanistan for 30 years. Think of it as nearly half a century of experience, all bad.

And yet, despite claims from the administration that the war is on its decline (claims which may or may not prove reliable and, even at best, will not result in a complete withdrawal of American involvement from Iraq), we still have a large military presence in Iraq, and many still contend that we need to continue it:

The Iraqis, so the argument goes, need us.... In the year to come, based on what we’re seeing now, such arguments may intensify. Terrible prophecies about Iraq’s future without us may multiply. And make no mistake, terrible things could indeed happen in Iraq. They could happen while we are there. They could happen with us gone. But history delivers its surprises more regularly than we imagine — even in Iraq.

In the meantime, it’s worth keeping in mind that not even Americans can occupy the future. It belongs to no one.

In short, yes, as the title of this post says, bad things may happen in Iraq if we leave.  But they will happen anyway if we stay, and at least if we're gone one good thing is certain to happen:  The U.S. will no longer be perpetuating an illegal, immoral, and unnecessary invasion which should never have occurred to begin with...well, at least it won't be perpetuating one of them.

Read the rest of Englehardt's analysis here.