There are two answers to the question of why the situation with the Houthis was going out of control.
The first answer relates to the number of missiles aboard a ship. US ships are relying on SM-2 missiles, part of the AEGIS system. One expert estimates the number available as follows:
“The [AEGIS] destroyers have a complement of 96 VLS cells, while the [Ticonderoga class] cruisers have 122. …However, they need to fit a mixture of weaponry in those cells so they can’t all be used for air defense.
The exact ratio of these weapons is largely dependent on the mission and the possible threats faced. However, at least 200 ESSM and another 100 or so SM-2 or SM-6 seems like a fair guess. Maybe a bit more.”
In short, each of the AEGIS has around 100 missiles.
The British Sea Viper air defense system is the main defense system HMS Diamond relied on to fire at Houthi drones and missiles. “Type 45 Destroyers, also known as Daring-class destroyers, are specifically designed around the Sea Viper (PAAMS) air-defence system.
Neither the US nor the British ships can be reprovisioned at sea, so they have a limited ability to “stay in the fight” if it continues for any length of time.
As the January 10 Houthi attack demonstrated, the Houthis were increasing the number of daily attacks. How large an arsenal the Houthis had, therefore, challenged US and British defense capabilities.
One needs to add that using missile defense is very expensive. Each SM-2 missile costs $2.1 million each. Sea viper, which can either be an Aster 15 or Aster 30 costs either £1m to £2m a time ($1.25 million to $2.5 million). Nor does this take into account the challenge of replacing these missiles, once expended. It not only will be more expensive, but could take years of production time.
This leads to the second answer: what happened that was different than before?
There are three possibilities. The first is that on January 10 the British ship HMS Diamond and US destroyers in the area were directly targeted by the Houthis. If this interpretation is correct, it meant that the Houthis decided to directly attack US and UK warships.
The second possibility was Houthi threats and the response by the US.
On December 20, the Houthi leaders warned “they would strike U.S. warships if the Iranian-backed militia was targeted by Washington.”
The Houthis response was “The American enemy bears the consequences of this crime and its military movements in the Red Sea to protect Israeli ships won’t prevent” the Houthis from “performing their religious, moral and humanitarian duty in support and aid of those who have been wronged in Palestine and Gaza.”
But the third reason is more important than shooting up some Houthi boats filled with commandos.
In their attack on January 10 the Houthis fired ballistic anti-ship missiles at US and British warships.
Previously the Houthis relied on Kamikaze drones and anti-ship cruise missiles. It is likely the drones used in the shipping lanes near the Bab-el-Mandeb straits are the short range type.
Yemen has a number of different types of anti-ship cruise missiles. This means they can be tracked by radar and destroyed with air defense missiles.
The problem is much more difficult if the missiles fired are ballistic missiles, meaning they are powered by a rocket motor. This makes them much faster and gives defenders far less reaction time. The Houthis have many different anti-ship ballistic missiles supplied by Iran. They present a serious problem in restricted sea lanes near the shoreline, which is the key problem facing the allies in the Bab El-Mandeb strait.