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The Siege Tank is a Terran mechanical unit that is able to be built from the Factory once it has an attached Machine Shop. The Siege Tank has two modes of attack: a tank mode, as well as a siege mode that has to be researched at the Machine Shop. Both modes are only capable of damaging land units. An unsieged Siege Tank deals 30 explosive damage, and one that is in siege mode deals 70 explosive damage plus splash. While in siege mode, the Siege Tank is incapable of movement and gains a range of 12, but also has a minimum range of 2, meaning melee units can attack without being attacked. The Siege Tank is able to freely transform between the two modes with a short transformation sequence. You should try to attack unsieged Siege Tanks because they will in all probability deal one or two less shots. If there are a few Siege Tanks, which most times there are more than that, it adds up exponentially quickly to do so.
A Siege Tank in siege mode has the greatest attack range in StarCraft. It is even greater than its sight range, so it benefits from units (e.g. lifted off buildings) that give it more sight.
Researched at the Machine Shop.
 Competitive Usage
 versus Zerg
Siege Tanks are used against Zerg to clear out Sunken Colonies in standard M&M Terran. They also prove very effective against Lurkers. Siege Tanks don't have to be sieged to be effective against Burrowed Lurkers. Surprisingly they outrange the Lurkers even unsieged. In the recent Mech Builds, Tanks are used if Zerg chooses mass Hydralisks over Mutalisks, as Siege Tanks are very effective against mass Hydralisks. In a standard Mech build, the Terran usually ends up with a mass of Tanks supported by Goliaths. The best response to this is Defilers with Dark Swarm and Plague. Still, Dark Swarm is not nearly as effective against mech as against M&M since Dark Swarm does not protect Mutalisks and Tank splash damage will still flatten mass Hydralisks. Plague is a greater threat, weakening Mech to Mutalisk attacks
Note that burrowed units under Dark Swarm take no damage from Siege Tanks, thus Mech will need Science Vessel support against Lurkers and Defilers.
 versus Terran
The Siege Tank is a staple of standard TvT. Many TvTs will involve lines of Siege Tanks in defensive positions, with Goliath, Wraith, Vulture, or even Battlecruiser support. Dropships are regularly employed to avoid the Tank's powerful ground attack and exploit its lack of mobility and an anti-air attack.
Early game, you can use 2 sieged Tanks and 2 Wraiths to kill an enemy Siege Tank in one volley, with Wraiths spotting for the Tanks and dealing the finishing blow at the same time.
Two Siege Tanks with a +2 attack upgrade will kill an enemy Tank with only one volley, regardless of its armor.
 versus Protoss
The Siege Tank is also a staple of standard TvP. Its 70 damage with splash decimates Dragoons and other ground units which compose a standard PvT army. In PvT Siege Tanks are usually accompanied by Vultures, which soak up damage and protect against Zealots.
Siege Tanks can also be used to disrupt mining. Because mining Probes have few Hit Points and are usually clustered together, a few siege mode blasts can devastate a Protoss mineral line. Siege Tanks' excellent range make them ideal for assaulting Protoss mineral lines from convenient drop points or across ridges or cliffs. Due to its energy shields, the Probe is the only worker unit that can be killed by a single unupgraded Siege Mode shot, even when upgraded to 3/3 armor/shields, making well-orchestrated tank drops far more devastating against Protoss mineral lines than they would be against other races.
You can use shift attack to spread out Siege Tank fire so the shots aren't wasted on overkills. If a lot of units like Dragoons are coming, you hold down shift and attack move spam on as many Dragoons as you can.
- Shift micro to spread out Siege Mode fire. Thread
 See also