The revelation that Coalition forces have discovered about 500 shells containing chemical weapons (mostly sarin nerve gas and mustard gas) since 2003, most of which are pre-1991 Gulf War vintage, leads to the question as to why the U.S. waited so long to reveal this. The U.S. government has taken a beating for supposed failures to find weapons of mass destruction in the press, and from political opponents. There have been some discoveries that have made the news, most notably an incident in May, 2004, when terrorists used a 155-millimeter shell loaded with sarin in an IED. The shell detonated, exposing two soldiers to sarin nerve gas (both of whom survived and recovered). It is this attack that provides one explanation as to why many of the finds have been classified.
If the United States were to have announced WMD finds right away, it could have told terrorists (including those from al-Qaeda) where to look to locate chemical weapons. This would have placed troops at risk - for a marginal gain in public relations. A successful al-Qaeda chemical attack would have been a huge boost for their propaganda efforts as well, enabling them to get recruits and support (many people want to back a winner), and it would have caused a decline in American morale in Iraq and on the home front.
The other problem is that immediate disclosure could have exposed informants. Protecting informants who provide the location of caches is vital. Not only do dead informants tell no tales, their deaths silence other potential informants - because they want to keep on living. A lack of informants leads to a lack of human intelligence, and the troops don't like being sent out on missions while short on intelligence - it's easy to get killed. This has led to media coverage (particularly around "milestone" deaths) and
The biggest danger with intelligence is in its over-use. This might sound odd, but it is the biggest concern many decision-makers in wartime have to make. Protection of an intelligence advantage can be so important that it might require allowing an enemy action to go forward (like the 1940 bombing of Coventry - Churchill allowed that to occur rather than risk exposing the British ability to read German codes), or it might require high-level approval of a mission (like the 1943 operation in which Thomas G. Lanphier shot down the plane carrying Admiral Isoroku Yamamoto - the decision to attempt the mission was made by the Secretary of the Navy). In the world of intelligence, decisions are rarely simple, and easily answered. A great deal of consideration goes into the decisions based on the intelligence provided, and when to release the information to the public. - Harold C. Hutchison (email@example.com)