Deir Ez Zor, the largest city in Eastern Syria, has since 2013 become the site of one of the world's longest and most prolific sieges since the Second World War. Since the beginning of the insurgency against the Syrian government in 2011 Syrian forces stationed in the city have clashed with growing numbers of Islamist insurgent forces. With the capital cut off and far away and the Syrian military overstretched, Syrian forces at Deir Ez Zor had little support to fight increasing well armed Islamist militants whose weapons were constantly replenished by their foreign backers and whose numbers continued to swell as jihadists across the world answered the call to holy war. By the end of 2013 Syrian troops were encircled by jihadist forces - holding only half of the city and a critical strategic airbase.
It was only in 2014 that the siege would truly begin to tighten as the expansion of the Islamist State (IS) jihadist group led it to occupy much of Eastern Syria. After defeating rival Al Qaeda affiliated Islamist groups around Deir Ez Zor IS began a far better organised siege against Syrian forces stationed there. All roads were closed and all fibre and electric cables were cut, leaving the defenders isolated and able to receive supplies only by airdrops and helicopters. Syrian positions appeared on maps as a small island in the vast sea of IS held eastern Syria - the only remaining outpost of the Syrian army in a vast region held by jihadist militants. Syrian Republican Guard Commander Islam Zahreddine, who led the defence, said their position was "like an island in the middle of an ocean - totally isolated" with the closest city being over 250km away (almost twice the width of the Taiwan straits.) Syrian forces were able to heavily fortify their positions however, while using the advantage of their numerous combat aircraft to strafe and bomb jihadist forces. Numerous IS offensives to capture the military airbase repeatedly failed and the militant group sustained heavy losses, though they used suicide bombings, tank assaults and the targeting of Syrian service members' families to attempt to gain an advantage.
From September 2015 the tide of the war began to turn as Russian air, ground, naval and special forces deployed to Syria in growing numbers. The Russian Air Force targeted IS positions throughout the country from supply depots to command posts - forcing the militant group to retreat and taking pressure off Syrian ground forces. Russian air and cruise missile strikes on IS positions surrounding Deir Ez Zor in coordination with the defending garrison proved a significant asset while air drops of food and weapons helped to bolster their resistance. IS nevertheless continued to mount new offensives, and as the city was in the heart of the militants' territory there was a limit to what even Russian support could achieve. In September 2016 the United States intervened directly against the Syrian Army, using its lethal A-10 ground attack aircraft supported by F-16 fighters to inflict over 200 casualties on Syrian troops stationed at Deir Ez Zor airport - a wholly unexpected attack for which Syrian forces were unprepared. Within an hour this was followed by a renewed IS offensive against the weakened defenders, and the jihadist forces succeeded in pressing their advantage and capturing the strategically important Tharda mountain overlooking the critical airbase. The United States for their part denied any coordination with IS and insisted that their attack, though it had gone on for over an hour and been carried out by low flying aircraft, had been unintentional. The Syrian government and its supporters alleged otherwise.
In 2017 in the face of rapid territorial losses on all sides of their self proclaimed caliphate, Islamic State forces attempted several desperate assaults on Deir Ez Zor. In January 2017 they were successful in cutting the road linking the military's airbase with the rest of the city, splitting the government controlled enclave in two. Civilian casualties were high during all of these attacks, and both militants and government forces sustained heavy losses. IS used the few dozen tanks they had to conduct armoured assaults against Syrian positions with some success. Attacks continued into July and IS continued to gain ground. It was only their mounting defeats on other fronts which prevented them from pressing this advantage further. With the Syrian Arab Army's victories in Raqqa and the Eastern Qalamun mountains in June, Syrian forces were able to press towards the besieged city to lift the siege on the defenders. Spearheaded by the elite Tiger Forces the Syrian army, supported by the Russian Air Force, continued to press closer towards the city and attack the besieging IS forces from a second front - forcing them to retreat in early September. On September 5th the siege was finally broken and the beleaguered population, which had long suffered malnutrition and near starvation under the IS blockade, again began to receive humanitarian supplies. The defenders of Deir Ez Zor had for years been a thorn in the side of IS, launching air strikes against its forces from fortified positions in the heart of its territory. The liberation of Deir Ez Zor from IS and their rapid territorial losses throughout the country indicate their imminent defeat in Syria. Syria's many foreign adversaries will respond to this victory, and whether they will take more direct action against the Syrian government, is yet to be seen.