Counter insurgency warfare within a civilian population works on many levels - whilst your hard core fighters may be very small - the IRA for instance never had more than 300 members "on active service" they would rely on support, either direct indirect in the form of food, weapons, shelter communications and intelligence from the local population.
They would be able to rely on not being informed upon to the security forces - the security forces in turn are not able to obtain information in sufficient quantities and of sufficient quality to remove threats.
For example - EOD ambush is set up at a roadside - the locals all know about it - they walk on the road every day, it's probably marked so they can avoid it - do they tell the security forces - nope they stay out of the fight not directly choosing sides but favouring the insurgents.
You need very few hard core fighters to tie down an army - as Mao said they use the population as a fish uses the sea - the population gives them everything they need.
In the last 150 years the ONLY case where insurgents were defeated was in Malaysia - and that was probably due mostly to the fact that the insurgents were Chinese imports and not locals.
America might think it will win this war but history is against you - start putting together a tasteful memorial, big enough for about 10,000 names and sweep the 75,000 odd maimed and wounded under the carpet - declare a wonderful victory - go home.