On Wednesday, US Defense Secretary Pete Hegseth said that Iranian air defences had been severely degraded over the course of the last four days and that the US military would begin to deploy 500-pound, 1,000-pound, and 2,000-pound precision gravity bombs.This announcement marked a major tactical pivot in the ongoing campaign against Iran, signalling a move away from cautious, “stand-off” ammunition strikes to critical direct bombardment in the US-Iranian war.Munitions used so farUntil now, the US has relied on expensive, long-range “standoff munitions”, which are launched from outside the range of Iran’s air defence systems and strictly dismantle adversarial early-warning radars and surface-to-air missile batteries.To reduce the possibility of pilot casualties by keeping them away from the conflict, this ammo is deployed from US naval destroyers and stealth aircraft.The Tomahawk cruise missile and the LUCAS (Low-cost Unmanned Combat Attack System) drones have been the front-running standoff munitions for establishing US-Israeli dominance over Iranian airspace in the past few days.Also in Explained | How the war in Iran could dent Russia’s aerial dominance in UkraineA gravity bomb, historically known as a “free-fall” bomb, is an unpowered munition. Unlike a cruise missile like the Tomahawk, it lacks an internal engine. Once released, its trajectory is dictated entirely by gravity, aerodynamics, and the speed and altitude of the dropping aircraft.Despite their terminology likening them to World War era relics, these variants are the modern workhorses of the US Air Force. Deployed heavily since the turn of the century in Iraq, Afghanistan and Syria (against ISIS), they have also been used by Israel in Gaza and Lebanon.Story continues below this adThe US has modernised these free-fall weapons into highly accurate “precision ammunition” by attaching Joint Direct Attack Munition (JDAM) kits. These tail sections, equipped with GPS and steerable fins, allow the bomb to glide precisely to specific coordinates.Conventional vs nuclearCrucially, the weapons Hegseth referenced are strictly conventional, packing hundreds of pounds of standard chemical explosives, cost roughly $25,000 to $30,000 with a JDAM kit, and are deployed by theatre commanders (typically four-star generals). The current campaign relies on the conventional Mark 80 series.The US arsenal also includes nuclear gravity bombs, such as the B61 and B83 series. The difference between these and conventional bombs lies in yield, cost, and authorisation.Nuclear gravity bombs measure their yield in kilotons or megatons of TNT. They cost upwards of $20 million per unit to modernise (along with expensive Life Extension programs) and carry the ultimate prerequisite: explicit authorisation from the US President, as their use would signal a global nuclear escalation.Why the shift?Story continues below this adThe shift from standoff missiles to conventional gravity bombs comes down to a trade-off between cost and risk. Missiles cost millions of dollars each but keep pilots perfectly safe. Gravity bombs are incredibly cheap and allow for high-volume bombardment, but they carry a massive catch: the pilot must fly directly over or very near the target to drop them. Therefore, deploying gravity bombs is only a viable tactic once a military achieves “air supremacy.” Hegseth’s announcement is a declaration predicated on the assumption that the Pentagon has neutralised the Iranian anti-aircraft threat.What is the US Air Force deploying now?The US stockpile of conventional gravity bombs is centred on the Mark 80 series, which will now be deployed with precision guidance. These munitions are highly versatile and can be dropped by a wide range of aircraft, from the F-15E Strike Eagle and F-35 stealth jets (tactical fighters) to heavy strategic bombers such as the B-52 Stratofortress. The Mark 80 series includes:500-pound (Mk 82): For “soft” targets such as light vehicles, radar installations, or exposed infantry positions.1,000-pound (Mk 83): A medium-yield weapon utilised for destroying reinforced concrete structures, command posts, or smaller bridges.2,000-pound (Mk 84): The bunker-buster, capable of obliterating deeply buried military complexes or large industrial facilities, creating blast craters up to 50 feet wide and 36 feet deep.