Alright, I mad this to outline why the HECU in Half-Life failed their mission in "cleaning" the facility of aliens and personnel:

1. The urban (for lack of a better term) and underground terrain of Black Mesa impaired the use of tank, air, and artillery support, greatly damaging the effectiveness that the soldiers could operate at. It doesn't take a military genius to figure out that modern (early 20th-21st century) combat requires the three things stated above to win against an enemy in warfare or else they will definitely loose (or suffer more casualties than necessary). This lesson was most prevalent at the Battle of Stalingrad, where the urban layout prevented the use of tanks en masse and was part of the reason the battle was lost for Germany. The lesson was also quite prevalent in the Pacific campaign, where Japan's lack of air support led to their defeat at many battles in the Pacific (after 1942).

2. The HECU was greatly outnumbered, as their forces of tens of thousands found themselves fighting tens of millions of hostile aliens. This is backed up by the fact that throughout Half-Life, Blue Shift, and Opposing Force, the sheer number of aliens increases rather than lower, and the number of HECU Marines decreases. Not to mention' at the areas where artillery, air, and/or tank support was possible, it was heavily implied that the aliens still attacked just as fiercely.

3. The size of Black Mesa can only compare to the size of Aperture Science, as both are said to stretch out for miles in all directions. Black Mesa has some of the most confusing and backwards layout and architecture in history, as shown by FM and SM (Shepard's Mind) and many Marines had undoubtedly gotten lost in the catacombs (made worse by the lack of maps and large amount of safety hazards).

4. The HECU Marines had little experience in urban combat, as implied by entries in Shepard's diary/journal (entries can be found here: http://sectorw.wikia.com/wiki/Adrian_Sh ... ry_entries).

5. The order to shoot unarmed civilians was probably demoralizing, and caused desertion to occur.

( This was largely speculative)

I gotta say, having hundreds of their forces mowed down by a certain Gordon Freeman certainly didnt help.

Well my computer was slowing down, and I hate the number 6.

So skip 6, and jump to 7... They used to do that with elevators and 13...

My computer still lagged horribly by #4.

You need to upgrade, or clean up your system then. How many tabs open, what OS, what browser, what programs running, what if any anti-virus, and what hardware?

PM me if you want to get your system optimized.

Good points there. I think it´s weird that the HECUs aren´t trained for urban enviornments. Since Half-Life takes place not too far in the past a special force like the HECU should be trained for urban enviornments, just like most spec ops these days. Also i would expect hazardous enviornment in urban areas like chemical factories ,a nuclear power-plant or Black-Mesa, rather than in the open field, that´s why a Hazardous Enviornment Combat Unit should be trained for those areas. Why send them in the first place if they are not trained for that operation, would be another question.

Another point i would add to your list is that they probably weren´t trained for alien combat.

You make a strong argment, but the reason I think that they weren't trained for urban combat is probably because these were recruits and that the CO thought it would be a walkover. They obviosly weren't being trained for alien combat, largly due to:

- Lack of information on them
- They were probably trained for an offensive into Eastern Europe during a possible WW3, or an offensive in some other country

HECU seems to be a bigger thing since they have all sorts of support as well as a big number of units and i think a good part of them is probably prepared to the things you mentioned, but it seems like shepards unit had specialised training for black mesa, you´ll find that in shepards diaries.

I think the lack of information is the bigger thing here. We know that the scientists knew Xen and the aliens before the resonance cascade (you can find various labs in HL1 and the add-ons where they do experiments with the barnacles and stuff), but you never see a vortigaunt (i think, maybe i´m missing something) or any of the other bigger, more dangerous aliens, i think they weren´t able to gather enought information on those yet and i think they won´t give Top secret information to soldiers anyways.

Also they didn´t expect such a big attack from Xen in case of a resonance cascade, because they weren´t aware of the nihilant, who coordinated the invasion rrather then just alines who randomly appear

Just remember that #1, #2, and #3 are the biggest reasons that they failed. The urban enviro doesn't offer many chances for air, artillery, or tank support (unless in defence mode), the superior numbers allow for human (or alien) wave attacks (and as a result, you don't have to devote as much time for proper planing), and, as demostrated by our Mind series, the confusing layout and lack of maps can send people into a life threatning safty hazard (thank you, Black Mesa).

I get your point. Well you can´t expect soldiers to survive the box-smashing room.

Or the nut cracking room.