October 9, 2016:
In September Iran displayed Ghadr (Qadr) H IRBMs (Intermediate Range Ballistic Missile) and was described as its latest long range (2,000 kilometers) ballistic missile design. Ghadr H was apparently more accurate and reliable. Earlier in 2016 Iran claimed to have successfully tested two Ghadr H and proved that this missile had overcame the problem of IRBM accuracy. While Iran talks about IRBMs armed with nuclear warheads, it will be years before such a warhead is available. Meanwhile Iran has high-explosive and chemical warheads. These can be a threat to Israel if their warheads are accurate enough to hit targets like a nuclear power plant or the part of an airbase where the most effective Israeli warplanes are kept. Meanwhile Iran has been building fortified storage and launch facilities IRBMs outside Khorramabad (near the Iraq border in the Zagros Mountains). The first one was called the Imam Ali missile base and is run by the IRGC (Revolutionary Guard Corps) as are all things having to do with ballistic missiles and nuclear weapons. The IRGC is led by and largely composed of Islamic conservatives and radicals who see nothing wrong with committing national suicide by firing IRBMs at Israel and risking nuclear retaliation.
Back in 2009 Iran got its first solid fuel IRBM (the Sejil II) to work. At the time Iran was discussing plans to build hundreds of Sejil IIs by 2015. This would include more mobile launchers for many of these for these missiles, to make them even more difficult for anyone to spot, and destroy. Solid fuel missiles are much more dangerous because they can be launched without much preparation. This is critical, as the older liquid fueled missiles take hours to prepare for launch, and spy satellites pass over Iran frequently enough to spot this. At the time Iran already had some liquid fueled IRBMs and production of these stopped once a solid fuel design was ready. There was no mass production of Sejil II either, mainly because of the lack of a nuclear warhead and thus the need for a more accurate guidance system to make non-nuclear warheads effective. That development process also caused the improved Sejil II to be rebranded as Ghadr H. Changing names like that is an Iranian thing that is mainly cultural.
The main reason Iran developed solid fuel IRBMs was because it needed that range to reach Israel. Iran is has also been working towards a longer range missile that can reach Europe. This is not speculation as North Korean missile component shipments to Iran had been intercepted. Iran began buying ballistic missiles and related technology from North Korea in the 1980s. It soon became a two-way trade because Iran proved more adept at refining North Korean designs to make them more reliable and efficient. It turned out that Iran and North Korea had been exchanging solid fuel rocket motor tech since the 1990s. Iran has been manufacturing solid fuel for smaller rockets since the 1990s and partnered with North Korea to develop the technology to build larger, and reliable, solid fuel rocket motors.
Iran had been producing Shahab 3 liquid fuel IRBMs since 2004, but this missile was basically 1960s technology with the addition of GPS guidance. Russian and North Korean missile technology has been obtained to make the Shahab 3 work. This has resulted in a missile that apparently would function properly about 60-80 percent of the time and deliver a warhead of about one ton, to a range of some 1,700 kilometers to within a hundred meters of where it was aimed. By world standards, this is a pretty effective weapon. A solid fuel version of this missile would be, if the solid fuel was of reasonable quality, about ten percent more reliable than liquid fuel, and easier to hide and launch. The greater accuracy (under 30 meters, preferably under ten meters) eventually became the missing piece for the ideal Iranian non-nuclear IRBM.
Israel has threatened to retaliate with nukes if Israel is hit with chemical or nuclear warheads. Israel has Arrow anti-missile systems that can stop Shahab 3s or solid fuel IRBMs, but only a few at a time. If Iran launched a dozen or more IRBMs simultaneously some would get through. If Iran had several hundred IRBMs they could launch most of them at Israel, using high explosive warheads, and do a lot of damage. Israel could respond with its own Jericho II missile, but this system was designed for use with nuclear weapons, and Israel apparently only has 20-30 of them. Israel could respond with air strikes, and cruise missiles from submarines in the Persian Gulf or Indian Ocean. But, again, this would appear as a limited response to massive Iranian missile attacks. An Iranian attack with nuclear warheads would kill a large number of Moslems, and even radical Iran might be put off by that, because Israel would likely respond in kind.
A large number of IRBMs can also be used to intimidate nearby Arab countries, as these missiles could damage oil production facilities. But Iran is apparently developing new shorter range (under 800 kilometers) range to be used against their Arab neighbors.