The October 8th explosion on the Kerch Strait bridge was apparently one of several attack plans developed by Ukrainian and British special operations personnel. This planning began back in April and several plans were developed. One involved cruise missiles hitting concrete pillars supporting the bridge where it consisted of a long span over the shipping channel. A similar attack plan called for divers to approach at night and place explosives on these pillars. In both cases the key central span would be destroyed and cut road and rail traffic for months while repairs were made. The Russians were apparently aware of this vulnerability and soon after the invasion began defenses around the central span were increased to defeat or disrupt missile or underwater attack. This did not deter the Ukrainians who continued to consult with their British counterparts on other plans for destroying key portions of the bridge.
Ukraine wants the bridge gone because it enables easy access to Crimea by the Russians and restricts the size of ships that can enter that portion of the Ukrainian coast.
In 2019 the railway link across the new Kerch Strait bridge connecting Russia with Crimea was opened for service. This enabled the bridge to move 13 million tons of freight and 14 million passengers a year. The bridge cost nearly $4 billion and is mainly a prestige and diplomatic project as it enables Russia to control the Sea of Azov. In April 2018 Russia declared the Sea of Azov, reached from the Black Sea via the 4.5-kilometer-wide Kerch Strait, was now under Russian control.
The Crimean Peninsula, when it was part of Ukraine, was separated from Russia by the Kerch Strait. Maximum depth of the strait is 18 meters (59 feet) and there had long been talk of building a bridge between Crimea and the Kerch Peninsula (now and always part of Russia). After Russia seized Crimea in 2014, proposals that a bridge be built actually turned into reality. The Kerch Bridge opened in March 2018, at least the highway part, the sturdier railroad section took longer to complete. With that Russia declared the Sea of Azov under Russian control and no foreign ship could enter with Russian permission. The Russians seized ships trying to reach the Ukrainian ports of Berdiansk and Mariupol that are on the shore of the Sea of Azov. Russia was putting these two ports out of business. Ukraine accused Russia of violating international law as well as a 1990s Russia-Ukrainian treaty that prohibited the seizure of Crimea or building the bridge and restricting access to the Sea of Azov. The problem with the bridge is that it only allows about 30 percent of the large bulk carrier ships that visit Ukrainian ports to get into the Sea of Azov. Taking down that portion of the bridge would reopen the Sea of Azov to large ships.
The October 8th dawn attack on the bridge was only partially successful. This attack involved a special bomb weighing several tons carried by a truck crossing the bridge. It was a suicide mission for the driver and the explosives went off short of where they were intended to detonate for maximum damage. It is unclear if this was a driver error or timing error on the detonator. If the bomb had gone off at another point, three deck sections would have dropped into the water rather than one. With one span down the Russians were able to make temporary repairs and resume limited vehicle traffic within a day. The explosion did ignite adjacent railroad cars carrying fuel. The light from these fires could be seen from a long distance. The fire weakened the structure but the Russians were unsure of how much. They continued running normal rail traffic. Fully restoring the highway span is taking longer, limiting vehicle traffic until it is completed. If three deck spans had gone into the water, vehicle traffic would have been halted for weeks and limited traffic only for months. Russian leader Putin was furious about this attack and regarded it as yet another unpredictable and effective Ukrainian innovation that Russian forces were unable to cope with. If the railroad portion of the bridge collapses, Russia is one step closer to losing Crimea to the advancing Ukrainian forces.
The rail line just to the north of the two road lines was much more important as only it could carry the vast quantities of fuel required by Russian aircraft based in the Crimea, plus the supply and armored vehicles in the Kherson area to the north. The Russians had declared the bridge essential for the defense of Crimea and the support of military operations north of Crimea. At the time of the bridge attack the Ukrainian offensive was making progress in capturing all the Russian occupied territory north of Crimea. Once that is done, the only supply route into Crimea will be the Kerch bridge. The Ukrainian advance also brings Ukrainian HIMARS trucks close enough to launch attacks on the Kerch bridge.
The new Russian commander of forces in southern Ukraine admitted that his depleted ground forces were unable to halt the advance. While admitting to the worst-case scenario, the new commander does have some units he can depend on and is deploying them to slow the Ukrainians down as much as possible. The recent “partial mobilization” declared by Russia in mid-September was only able to put about two-thirds of the men it expected into uniform. Most of those new troops were dismayed to find themselves sent to Ukraine after a few days or weeks of training and with inadequate equipment. When these new troops reached southern Ukraine, they found the supply situation worse and they were assigned to depleted (by casualties and desertion) units led by uninspiring officers. These are the units that are unable to halt the Ukrainian advance. Russia is putting some new troops through longer training but the equipment shortage is everywhere and few Russian troops are really enthusiastic about fighting to keep Ukrainians from taking back their territory.
While the recent Kerch bridge attack was not a complete success, it was demoralizing for many Russians who had been told a successful attack on the bridge was unlikely and the Ukrainians were not going to take back as much territory as they have in the last ten weeks.