When the rebels finally reached Bashar al-Assad’s sprawling palace in Damascus, the gates were open. There was no traffic on the floodlit highways leading into the vast estate, and apparently no defenders among the carefully tended trees. In the empty guardhouses, coats were still hung on the back of doors, so swiftly had the occupants fled.

Asked by the Guardian early on Sunday morning what had happened to the tens of thousands of men of the armed forces, pro-government militias, intelligence services, police and others who had all been dedicated to the preservation of Assad’s rule, one veteran Damascus-based analyst gave a terse answer: “They have vanished. Every single one.”

The rebel coalition that has stormed into the Syrian capital, forcing the departure of Assad after 13 years of horrendously costly civil war, had prepared meticulously for its offensive. A year ago, Hayat Tahrir al-Sham (HTS), the dominant force, established a military college to plan and organise the necessary resources.

The HTS-led coalition’s first attacks from its stronghold of Idlib in the north-west a week ago were on Aleppo, Syria’s second city and commercial centre. Videos the rebels posted on social media showed images of government armoured vehicles and tanks destroyed at negligible cost by drones, an entirely new capability developed over the previous year or so.

Equally important were units of shock troops known as the “red brigades” given extra instruction and conditioning before being deployed as highly mobile units behind enemy lines. Many members have survived years of fighting and are some of the most extremist, as well as most experienced, elements in the rebel coalition.

“The red brigades have six months training and are often veteran fighters who are highly ideological. In Aleppo, they were sent in ahead for sabotage operations and assassinations of regime officials,” said Broderick McDonald, an associate fellow at King’s College London.

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As the rebel advance gathered momentum, huge quantities of heavy weapons, tanks and even advanced missile launch systems fell into rebel hands. That they were incapable of using much of it did not matter particularly. Any fighting that did take place involved little more than assault rifles, rocket-propelled grenades, vehicle-mounted heavy machine or anti-aircraft guns and mortars. Most of it was over very quickly.

Yet the various strengths of the rebels were far from the decisive factor in the battle for the country. It was the weakness of the regime’s military and auxiliary forces that sounded the death knell for Assad’s rule and made the leader of HTS, Ahmed al-Shaari, better known as Abu Mohammed al-Jolani, the most obvious immediate contender to be the de facto ruler of Syria.

For many years, analysts have said that the Syrian army could “no longer be considered a cohesive force, but rather a coalition of regular forces and allied militias”.

Even these had been “hollowed out” by corruption, communal or sectarian favouritism and simple incompetence. The most senior posts were often reserved for members of Syria’s Alawite community, a heterodox branch of Shia Islam, from which Assad himself comes. The commanders of elite units had become involved in massive drug smuggling and other highly lucrative business ventures. Many of the footsoldiers were reluctant, frightened conscripts. A last-ditch effort by the regime on Wednesday to boost morale by hiking military salaries by 50% had no obvious effect.

Perhaps the principal reason for the swift military victory of the rebels was political: the careful and apparently successful outreach to Syria’s diverse communities, including some that had previously been supportive of the regime. H A Hellyer, of the Royal United Services Institute, pointed out last week that Ismaili Shias appeared to have been won over to the rebels’ cause, or at least not opposed it. Sanam Vakil, of Chatham House, described a “groundswell of support” for the HTS-led offensive which exposed the “brittle nature” of Assad’s regime. Even in Latakia, an Alawite centre, there were joyful celebrations on Sunday as residents dragged a statue of Assad through the streets.

This meant Assad’s troops had little to fight for. In Aleppo, the rebels promised to recruit rather than punish former soldiers. Musab Muslamani, one of those former soldiers, told TV reporters the situation was “good” as he queued to sign up. Such statements, viewed by his counterparts elsewhere in Syria, will have convinced many that their best option was to quietly slip away from bunkers and barracks.

A day or so later, in the far east of the country, though ordered to withdraw to Damascus to concentrate on the capital’s defence, thousands of soldiers chose to flee with their vehicles into Iraq. The former Nato supreme allied commander Gen Wesley Clark told CNN that the scenes showed the “collapse” of an army.

When rebels had come close to victory in Syria almost a decade ago, Assad’s military forces received vital reinforcements sent by Hassan Nasrallah, the then leader of Hezbollah, on the insistence of Tehran. Thousands of other fighters from Shia militia recruited specially for the task by Iran were sent too. The heavy and indiscriminate firepower of Russia, which deployed its air force, regular troops and the infamous mercenaries of the Wagner group to Syria, was crucial to turning the tide in the regime’s favour.

None of these resources were available this time. Moscow is distracted by Ukraine, Hezbollah greatly weakened by its war with Israel and Iran now on the defensive partly as a consequence. This meant Assad’s poorly trained, under-equipped and demoralised forces were left to fight it out with a highly motivated, competently led rebel alliance that had the active or tacit support of much of the population. In such circumstances, there was only ever going to be one winner.