The conclusion in a secret US defence department report that Belgrade had a spy in Nato in the early days of the Kosovo conflict backs up suspicions raised at the time by other military sources and raises the question of how much it affected allied operations and the conduct of the war.
Nato commanders, not to say political leaders, became increasingly frustrated by their inability to destroy Serb units carrying out atrocities on ethnic Albanians, precisely the activity Nato's military action was designed to prevent.
Nato's initial targets - those which Belgrade knew about in advance, according to the US reports - were mainly fixed sites, including anti-aircaft missile emplacements and military barracks.
The Serbs were reluctant to "lock-on" their missiles to Nato aircraft since this would make them vulnerable to counter-attack by Nato pilots. However, allied pilots throughout the 78-day bombing campaign reported attacks from surface-to-air missiles, fired, it seems, without their base being identified.
The first phase of the Nato campaign did not include targets over Kosovo - indeed, it was assumed that Slobodan Milosevic would put up his hands before new targeting plans would need to be agreed.
It was known that Yugoslavia had built an extensive system of underground shelters for its troops, guns and aircraft - a relic of President Tito's era, in preparation for a possible invasion by the Soviet Union. Many of its camouflage techniques, including the use of dummy weapons emplacements, the Serbs had learned from the Russians.
General Nebojsa Pavkovic, commander of the Yugoslav army in Kosovo, tells the BBC's Moral Combat programme this Sunday: "We used other measures, too: camouflage, decoys, and it was mainly these that Nato aircraft destroyed."
Nato pilots found small mobile targets so hard to find that on some days they dropped half their bombs on so-called "dump sites" known to be empty.
General Mike Short, commander of the allied air forces, said: "The Serbs dictated the pace of events, they dictated the battle rhythm. They came out to burn villages when they wanted to, they hid when the weather was good."
The question is how much did access to Nato intelligence give the Serbs warnings of allied attacks on specific targets - possibly even enabling them to place Albanians in military buildings and military vehicles in civilian convoys in areas they knew were going to be attacked.
But Nato did not start to strike targets in Kosovo until later in the war. In the first weeks of the conflict Nato commanders were frustrated by something the Serbs could do nothing about. Bad weather persistently prevented British laser-guided "smart" bombs from identifying targets.
And Nato's mistakes - killing civilians in urban areas and on Kosovan roads and railways - occurred mainly in good weather. Critics of the bombing campaign also said mistakes were the inevitable result of Nato's policy of restricting its pilots to bombing from 15,000 feet or above - a policy dictated not least by political considerations, notably Washington's determination to avoid horrified public reaction to the sight of "bodybags" coming home.
Failure to identify targets on the ground in Kosovo led Nato commanders - after fierce arguments between member governments - to strike economic and industrial targets, including power stations and the Serbian television headquarters in Belgrade.
Military commanders, in the first few weeks of the war, were cautious about what their bombers had achieved. Swept on by the rhetoric of their political masters, however, they began to make claims which they were to regret.
By early May, Nato was claiming that its aircraft had destroyed more than 200 tanks and had cut off Serb forces in Kosovo from their supply bases. It portrayed a Serb army whose morale was crumbling from mounting casualties, shortages of food and fuel and lack of sleep, as it dispersed into smaller and weaker units to escape the relentless bombing.
After conceding that the initial war aims - which were to avert a human disaster, as George Robertson, then defence secretary, put it - had failed, Nato claimed it was progressively destroying the Albanians' tormentors.
Yet when the western media saw the Serb military withdraw from Kosovo in early June, they saw convoys of Serb tanks, armoured cars, guns, trucks and military equipment untouched by Nato's air assault.
Nato's bombing campaign, with thousand of sorties and the dropping of tens of thousands of bombs, including sophisticated precision weapons, succeeded in damaging just 13 of the Serbs' 300 battle tanks in Kosovo.
A Nato spy could have provided Belgrade with crucial information, but the implication of the secret US reports is that two weeks into the conflict he or she no longer had access to allied targeting secrets.
His or her presence would certainly have hit the morale of Nato's commanders. But he or she is unlikely to have had any impact on the later conduct of Nato's campaign. This was determined by broad political necessities and military shortcomings.
Nato's final tally of hits and misses
Nato planes flew more than 3,000 bombing missions, for the loss of one plane to enemy fire, during the 79-day war from late March to June last year
After 1,955 of those missions, pilots reported hitting their targets (some claims were duplicates)
93 Yugoslav tanks were hit (out of a rough total of 600) - 26 of them "catastrophically" destroyed and 67 severely damaged
153 armoured personnel carriers were hit (out of an estimated 600)
389 artillery and mortar pieces were struck