Lycaeus Wrex wrote:Taros and Anphelion from IA are two prime examples of the Imperium getting soundly beaten by the xenos.
If you read those books you should already know the reasons why they were beaten.
Initially, the Imperium went to Taros unprepaired for the Tau. If I remember correctly they expected only an upstart governor and a poorly armed local population. After they discovered what they were dealing with, and even then I don't think they were completely aware of the full force of the opponent they were dealing with, they only sent in what they could (and wanted to) muster within a reasonable distance and time from Taros.
But the main reason probably, which is clearly present throughout the whole book, was the complete and utter strategic failure by the Imperials in command of the operation. I would have to read it again to provide details and alternatives, but their whole plan of landing in the desert and going all that way through it towards their target cities, creating rediculously long supply lines exposed to continuous hit&run ambush attacks, including their crucial water supplies, was almost too foolish to make the story plausible.
In the end the Imperium did not deem the planet important enough to reclaim immediately. If they would take the time and the effort to commit the right forces they would surely be able to win back the planet.
Anplelion was another case of not knowing what awaited them and being overwhelmed and outnumbered.
Reading that story gave me the impression the Tyranids could just morph into anything that suited them at that moment, giving them the advantage in all the different situations presented to us in the book, and doing all of this overnight. I did not like the story much to be honest. I thought it presented the Tyranids to us in a very unrealistic over the top way that made me think "yeah right, whatever". But again, I would have to read it again for the details.