John Keegan thinks clearly about the use of intelligence in war in the London Telegraph. Although referencing the Blair issue, it applies everywhere. A couple of good paragraphs:
"An even more striking example of disagreements, bearing directly on the current Iraq controversy, was over intelligence of German secret weapons. A strange leak, the Oslo report, had warned the British in 1940 that Hitler was developing pilotless aircraft and rockets. It was ignored until, in 1943, reports from inside occupied Europe referred to the subject again."
"A committee was set up, chaired by Duncan Sandys, Winston Churchills son-in-law. Its findings were reviewed by another committee, of which Lord Cherwell, Churchills scientific adviser, was the most important member. Cherwell absolutely denied the possibility of Germany having a rocket, and produced the scientific evidence to prove it. He persisted in his denial throughout 1943 until June 1944, when remains of a crashed V2 were brought to Britain from neutral Sweden. Shortly afterwards, the first operational V2 landed on London. Churchill was furious. Weve been caught napping, he burst out in Cabinet."
"Worse than napping. More than 1,500 V2s landed on London, killing thousands, at a time when Hitler was also trying to develop a nuclear warhead. The whole pilotless weapons episode demonstrates that, even under threat of a supreme national crisis, and in the face of copious and convincing warnings, intelligence officers can disagree completely about the facts and some can be 100 per cent wrong."