They can, just not for nation-states.
Arabs only fight well when its for God or clan. They fight much worse when a dictator forces them to. Otherwise, they don't fight at all.
— ♔ak (@akarlin88) June 13, 2014
(The Arabs are an extreme case, but really, it applies to virtually all Muslims).
Latest case in point: Taliban Fighters Capture Kunduz City as Afghan Forces Retreat
No matter how many gazillions of dollars the US pours into training them, no matter how many shiny toys they get from Uncle Sam, no matter by how much they outnumber the enemy (at least on paper), both Iraqis and Afghans alike collapse under the onslaught of men who fight not for gold but for God.
It’s not a failing of their trainers. The Soviets couldn’t do much either. (In both 1967 and 1973, the Arab armies had more military capital than the Israelis, and their tech was not inferior. But they still got whooped).
There have been some very good socio-political analyses of why this is the case, but ultimately it likely comes down to HBD reasons. Muslim, and especially Arab, societies tend to be based around consanguineous, which results in ultra-high levels of clannishness. The clan becomes the first focal point of loyalty. The second focal point is the Ummah, the wider Islamic community under God. This leaves precious little room for any strong emotional attachments to the third focal point, the nation-state that Europeans and East Asians are both intimately familiar, but which is quite novel, strange, and foreign to most Muslims.
Muslims who fight for states, to be of any use, have to be either amply compensated with gold (which tends to get stolen anyway), or be driven to do so by the fear of punishment from a dictator. This is why both Saddam’s Iraq and Assad’s Syria, for all their problems, made vastly better showings against their enemies than the militaries of semi-democratic modern Iraq and Afghanistan.
With a few exceptions like Trevor Dupuy, US analysts’ predictions of their own casualties during the Gulf War veered into the tens of thousands, whereas in actual fact they ended up being less than 300. In contrast, they systemically underestimated the difficulty of pacifying the conquered territories in both Afghanistan after 2001 and Iraq after 2003.
One explanation for this is well known to military theorists: In 4GW warfare, insurgents have the ability to fade into the general population, which means that the US (or the USSR in Afghanistan) is practically unable to make use of its gargantuan superiority in military capital. What use is a B-2 bomber against an IED? Effectively, it mostly comes down to the combat effectiveness ratios of US soldiers vs. insurgents, and the latter tend to additionally have the advantage of surprise in any engagement.
But all this should in principle be accounted for. What they might not necessarily account for, however, is the fact that insurgents – being far more driven, fighting for clan or God – have much higher combat effectiveness than the sorts of poor demoralized grunts they’d have steamrolled during the initial invasion. And from which they might have logically extrapolated to any insurgents, on the logic that these societies resemble the European ones that they would be most intimately familiar with (most recently in Serbia!).
When you have national Iraqi and Afghan armies fighting insurgents, you get not a double, not even a triple, but a quadruple whammy. A negative modifier due to the usual advantage insurgents have in surprise and concealment. Another negative modifier due to their status as soldiers in a national army suffering from all the typical problems of Muslim state armies, fighting insurgents who fight for God. And a final negative modifier due to them being democracies, if somewhat half-assed ones. I recall the Iraqi PM promising to execute officers who abandoned their stations to flee the Islamic State, but nothing came of it so far as I’m aware. Assad would have just gone ahead and done it.
The universalist American impulse to disregard human cultural and socio-biological differences not only makes it easy for the neocons to manipulate them into idiotic and irresponsible military adventures abroad. It also ensures that as soon as they leave, any political structures they leave behind soon get swept away as well by the irresistable tides of Anon (Nature or Nature’s Allah, to steal from the NRx lexicon) and the black flags of the Islamic resurgence.