One common complaint about the TEC series is that the Marines (as depicted by me) are unreasonably good, both militarily and politically. However, while I concede that the Marines are good, I don’t think that they are unreasonably good. And I certainly think that their competence is justified in-universe.
In recent years, we have seen Special Forces pull off astonishing feats in combat; the raid on Osama Bin Laden, for example. If that had been depicted in a novel, would that have been considered unreasonably good? The SEALS flew roughly 167 miles into Pakistani airspace, carried out a very brief and successful raid and then pulled out again – before any Pakistani military forces could intervene. No SEALS died in the operation, which was a great tactical success. Are the SEALS fictional? <Grin>.
The Terran Marine Corps takes a great many traditions from the USMC – however, it is not an exact copy of the real-life Marine Corps. In some ways, it is actually closer to the Royal Marines. The Marines depicted in the three books are, in our terms, Special Forces; they undergo two-three years of training to become Marines. By our standards, that is surprisingly long; recruits for the USMC take around 6 months to become Marines.
In real life, the quality of military units can vary radically. Both the 2nd Marine Expeditionary Brigade and the 507th Maintenance Company took part in the Battle of Nasiriyah (aka Ambush Alley). The 507th Maintenance Company, in short, got lost, accidentally entered enemy-held territory and ran into an ambush, a mistake attributed to human error. The Marines had to rescue them. If that had happened in a novel, would it be considered unrealistic?
[I’m comparing apples and oranges here, but my point includes that; military units are not created equal.]
When considering promotion within the Terran Marine Corps, it is important to note that promotion is only done from within the Corps – and any Rifleman who wants to be considered for promotion has to have served for at least a year on the ground first. The Grand Senate cannot send an officer from the Imperial Army, no matter how accomplished, to take command of a Marine unit. That’s laid down in the Marine Charter. What this means, in effect, is that the Marines have a certain immunity from political interference – and that any officer who does get promoted knows the basics of his job. (Anyone who has worked in a large organisation will have met plenty of people who are living embodiments of either the Peter Principle or the Dilbert Principle.)
Marine training and deployment patterns are designed to be crucibles. Those who succeed are part of a band of brothers; loyalty, honesty and determination are burned into those who graduate from the Slaughterhouse. They are expected not to let their fellows down – and many of those who might break often do so at the Slaughterhouse, rather than in the field.
This is explicitly NOT true of the Imperial Army or the Civil Guard. Officers are rarely promoted from the ranks; instead, they go to training schools and promotions are often determined more by political connections than actual competence. If this seems far-fetched, it is worth noting that it was partly true of the British Army in the pre-WW2 period. The system produced Nelson and the Duke of Wellington, but it also produced Percival (lost Singapore) and Hamilton (lost the Dardanelles, although how much of that was his fault is debateable.) It also was reluctant to make the best use of commanders from lower social classes or, for that matter, from the Colonies. (What if General Braddock had taken George Washington completely under his wing?)
That said, it is worth noting that the Marines in both TEC and WTBB have to deal with the worst of the Civil Guard. Even so, units of the Avalon Civil Guard performed creditably in the fighting on Avalon. Most of their problems were caused by political interference (or matters related to political interference) and outside their control.
Your mileage may vary, of course.