In general, as briefly as possible, then, looking at this whole Yemeni epic, the success of the Houthis, for me, revolves around two main factors:
There are many Houthis, you can easily have several hundred people in tiny areas, followed by more and more.
I have already talked about it, but I will repeat it - the desire to fight, that is, the desire to kill and the willingness to die.
Everything, everything else comes and goes.
Somewhere there are smart tactical moves on the part of the Houthis, somewhere there is the mediocrity of the pro-Saudi and Yemeni forces.
But all this pales against the background of these two factors: there are many Houthis, and they want to fight.
As we know, fire is not extinguished by water; A fire is only extinguished by an even larger vortex of fire.
That is, the mass of the Houthis requires another counter mass.
Those ISIS terrorists were crushed not by airstrikes (it was an important support, but not the main factor), but by the masses of Kurds, who were many of them and knew how and wanted to fight.
Even then it was no easy task, and IS drank a lot of blood, drained and retreated.
Iran's investment in the Houthis is probably one of the few truly competent, far-reaching and strategically calibrated moves.