BAGHDAD - US Defense Secretary Chuck Hagel arrived in Baghdad on an unannounced visit on Tuesday, as the United States expands its presence and touts momentum against Islamic State militants four months after starting a campaign of air strikes in Iraq."This is a long term effort. It's difficult. There will be setbacks. There will be victories. So I think that's where we are and I look forward to getting some first-hand assessments," he told reporters on Monday before his visit.