[1] Beginning in 2002, he began playing with the U.S. U-23 national team as it prepared for the 2004 Summer Olympic qualification campaign.
A reserve for much of the year, Warren was forced into action late in the season when starter Nick Rimando suffered a season-ending ACL injury.
However, shortly into the season, a string of mediocre performances by Rimando led Peter Nowak to give Warren another opportunity to seize the starting position.
He spent the rest of the season with the New England Revolution, as an injury replacement for the injured Adin Brown.
Due to the consistency of Reis, Warren did not appear in MLS play in 2006 or 2007, although he remained the Revolution's second-choice keeper.