Yemen's chaos is quickly becoming a catastrophe, for its people and the future of the country. As a kaleidoscope of factions are fighting for turf, some Yemenis have become so desperate they have fled by sea to Somalia, of all places.

Can the Houthis hold out against Saudi firepower?

So far they have. The Saudi-led "Operation Decisive Storm" has not yet been decisive. Many airstrikes have targeted Houthi positions in and around the port of Aden, where militia known as Popular Committees and some army units hostile to the Houthis are holding out against the rebels.

They are clinging on to a peninsula surrounded on three sides by the sea. The Saudis have air-dropped weapons and communications equipment to the Popular Committees, and a military spokesman says "We hope in a few days they will control most of the city."

A Saudi source told CNN's Nic Robertson that Houthi forces in the city had been cut off from resupply by land. That could shift the balance, but for now the Houthis, a Zaidi Shia minority in Yemen, are proving a tough adversary. Supported by some Yemeni army units, they want to deny the Saudis a bridgehead for ground forces and an enclave to which President Abdu Rabu Mansour Hadi might return.

Elsewhere in Yemen, Saudi officials say airstrikes have degraded Houthi-controlled military infrastructure, including key buildings in the capital Sanaa. The Royal Saudi Air Force, supported by other Arab states, controls the skies over Yemen. But as conflicts in Iraq, Libya and elsewhere have shown, airstrikes alone rarely inflict defeat.

In what may be a sign that the Saudi-led coalition has some way to go before getting the upper hand, Pakistan's Defense Minister Khawaja Asif said Monday that Riyadh had asked for planes, warships and troops.

So will the Saudis mount a ground invasion?

Adel al-Jubeir, the Saudi envoy in Washington, said obliquely "We don't have formal troops in Aden." Asked about the possibility of a broader ground campaign, he said: "No options are off the table, but I don't think we are there yet."

But Saudi Arabia may already have boots in Yemen, according to a Saudi source who told CNN's Nic Robertson that "non-combat" Saudi special forces were on the ground in Aden, helping to coordinate movements and the arrival of communications and weapons supplies.

The source also told CNN that special forces have been in action against Houthi units that seized the island of Mayun in the Bab Al Mandab Strait, one of the world's maritime chokepoints. One reason Saudi Arabia and allies such as Egypt are concerned about Yemen is that its coastline overlooks one of the world's busiest shipping routes through the Red Sea to and from the Suez Canal.

A concerted ground offensive to win territory across Yemen would be hazardous. The Ottoman Empire tried to pacify the region in the second half of the 19th century, only for it to become known as the "graveyard of the Turks." Then too, it was the Zaidi Shiites who came out of the highlands to lay siege to Sanaa.

Today the Houthis have the advantage of local knowledge in the mountainous north, which the Saudis would have to penetrate if they wanted to threaten the capital. They also have plenty of battle experience, after six brief wars against former President Ali Abdullah Saleh. Much of the country is well suited to guerrilla warfare. And Saudi intervention on the ground would galvanize jihadist groups such as al Qaeda in the Arabian Peninsula (AQAP) and a budding affiliate of the Islamic State in Iraq and Syria (ISIS) -- both of which are dedicated to overthrowing the House of Saud.

Egypt, which is closely associated with the Saudi action in Yemen, knows the price of intervention there. President Gamal Abdel Nasser sent in troops and planes in the 1960s while supporting republicans against monarchists in one of Yemen's many civil wars.

Egypt lost 10,000 men over five years, and history may not be lost on Egypt's current military strongman, Abdel Fattah el-Sisi. "Yet the temptation to seize this opportunity to restore Egypt's diminished stature in the region must be great — as it was for Nasser in 1962," says Jesse Ferris in Foreign Policy.

If Houthi forces succeed in taking Aden, and retaining control of the capital and Taiz, observers say the Arab coalition may have little option but to use ground troops.

Is there any hope of negotiation?

For the last three years, United Nations-led talks on power-sharing and a new constitution for Yemen have sputtered on. Time and again one faction accused another of bad faith. The Houthis learned that in the absence of state authority or other "power centers" they could take much of Yemen by force.

But they may also be aware of the dangers of over-reach. They are widely hated as interlopers in the south, where an independence movement is gathering steam. They may see military success, and temporary possession of Aden, as a bargaining chip for talks that would leave them the dominant power in north and central Yemen.

One Houthi official, Saleh al Samad, suggested at the weekend that the south could become autonomous and that talks could begin once the Saudi-led air campaign was halted. But he told Reuters that negotiations should be led by "non-aggressive" parties, presumably the U.N. For their part, the Saudis have floated a resumption of talks led by the Gulf Cooperation Council, a non-starter for the Houthis.

The question is: with whom would the Houthis negotiate? According to many analysts, President Hadi is now a spent force, having first fled the capital and then Aden. Hadi has tried to display his authority, firing military commanders while in exile, but effectively has little influence on the situation.

Any talks would be further complicated by the lurking presence of former President Saleh, himself a Shiite, like the Houthis. After being harried out of power in 2012, Saleh forged a marriage of convenience with the Houthis in an effort to regain influence, and many army units remain loyal to the ex-President. But there is a legacy of mistrust between Saleh and the Houthi leadership.

For now, none of the factions involved in the conflict seem ready to sue for peace as they jockey for military advantage, while civilians bear the brunt of the conflict.

Yemeni civilians without power, water, food

At the weekend some 16 million Yemenis living in areas under Houthi control, including the capital, were without electricity. The International Committee of the Red Cross (ICRC), which is trying to fly in relief supplies, says that "hospitals and clinics treating the streams of wounded from across much of Yemen are running low on life-saving medicines and equipment."

Even before the conflict escalated, Yemenis subsisted on an average of $7 a day. In contrast, their Saudi neighbors live on $86 a day.

The U.N. Childrens' Fund estimates that 100,000 people have been displaced across the country in the last two weeks, while Oxfam says that more than 10 million Yemenis do not have enough food to eat -- including 850,000 malnourished children. Over 13 million people have no access to clean water.

An urgent call by the ICRC for an "immediate humanitarian pause" to allow humanitarian supplies to reach the most vulnerable has gone unheeded.

The situation is especially dire in Aden, where most residents have no water or power, and long lines have formed to fill jerry cans with clean water. Cut off by the fighting, the city is short of food and basic supplies. The ICRC says dozens of people have been killed in recent days and the "streets of Aden are strewn with dead bodies." Multiple photographs show damage to apartment blocks in the city, whether from naval bombardment on Houthi positions, or from Houthi artillery strikes.

Some locals are clambering into boats and heading for Somalia, hardly a safe haven.

Al Qaeda thriving amid the chaos

Al Qaeda in the Arabian Peninsula (AQAP) has resisted a U.S. drone campaign and Yemeni armed forces for the past three years to retain a presence in eastern and southern provinces such as Hadramaut, Marib and Shabwa.

Now that the U.S. presence at the al-Anad airbase in the south is gone (evacuated days before the Houthis arrived) and the Yemeni army has effectively fallen apart, AQAP has far greater freedom of action. Its fighters briefly overran the coastal city of Al Mukallah last week -- freeing some 300 prisoners there before being pushed out by tribal fighters.

A senior AQAP member, Khaled Batarfi, was among those sprung from jail and has since appeared at a presidential palace in Al Mukallah.

AQAP is using the Houthis' advance southwards to rally Sunni tribes and expand a campaign of bombings and ambushes in places like Taiz and al-Bayda, where it claimed to have killed and wounded dozens of Houthi last week. As the conflict in Yemen takes on a more sectarian shape, AQAP will cast itself as the protector of Sunni tribes against what it calls "the Houthi rafidi [Shi'ite] religion."

The dangers of a regional conflict

Saudi Arabia sees the hand of Iran -- its great regional rival -- in the collapse of Yemen. Along with President Hadi, it has accused Iran of providing the Houthis with military aid and expertise -- charges that the Houthis and Iran both deny.

At the same time, Saudi Arabia's suddenly muscular approach appears to have ushered in a new foreign policy, according to Ray Takeyh at the Council on Foreign Relations in New York. "For Riyadh, the alliance with Washington is proving too unreliable in a region where sectarian conflict and collapsing states have ratcheted up tensions," Takeyh wrote in the Wall Street Journal.

The Saudis appear to have little faith in a U.S. administration averse to foreign entanglements and ready to compromise with Iran over its nuclear program. They look anxiously at the sectarian chaos engulfing Syria and Iraq.

So now Riyadh appears to be cashing some chips, appealing to Pakistan to join the coalition fighting in (or over) Yemen.

Pakistan receives huge amounts of aid from Riyadh -- last year $1.5 billion (in loans) -- and Prime Minister Nawaz Sharif spent time in exile in Saudi Arabia after he was overthrown in a coup 16 years ago. But it also shares a long and combustible border with Iran -- whose Foreign Minister, Mohammad Javad Zarif, is due in Islamabad on Wednesday. Yemen's troubles are already causing waves far from the Arabian Peninsula.

It is possible, as Rami Khoury puts it, that Yemen is the exclamation mark at the end of an era when regional states looked to Washington or Moscow, appealed to the U.N. or called an Arab summit to address issues in the neighborhood. Now countries like Saudi Arabia and Egypt are building ad hoc coalitions in the belief they can protect their own interests.

Yemen will test whether that belief is misplaced.