A two-square-mile grid of central Beirut offers a clue to Lebanon's troubles. Dominating the city's western quarter, the Sunni Mohammad al-Amin Mosque casts a glow over the concrete expanse of Martyrs' Square. About a mile south, through narrow streets, the Shia al-Hassanein Mosque rises up. Not far from here is the Druze temple, a glass and breeze-block building that looks like a public library, and on Mount Lebanon, the city's snow-capped backdrop, stone crucifixes dot the skyline.

The country's four main faith groups - Sunni, Shia, Druze and Maronite Christian - are imprinted on Beirut's landscape, just as their conflict is imprinted on Lebanon's history.

Over the past year, Lebanon has seen one government collapse while Hezbollah, backed by Iran and Syria, has grown in influence.

Many fear a new civil war as the country suffers its worst political crisis in a decade. The Arab spring that bloomed across the Middle East has yet directly to touch Lebanon, which has issues that are too entrenched and too complex to be resolved through the catharsis of a single revolutionary act.

Lebanon's problems are systemic and chronic, created by a political settlement born of empire. The country began as a French mandate, carved from a chunk of Ottoman Syria. With indepen­dence in 1943, its sectarian character was accommodated within a "confessional" political system that distributed office according to religion. The majority Maronite Christians were given the most important government positions (including the presidency), then the Sunnis, the Druze, and finally the Shias. In a country unable to function without consensus, it has served as a prophylactic against dictatorship for almost seven decades. But it contains grave flaws. Most egregiously, France's imperial cartography left aggrieved Syrians believing that Lebanon was theirs. Syria occupied the country between 1976 and 2005 and, through manipulation and political assassinations, has acted as a bacterial agent of instability there to this day.

Then there are the demographics. In the late 20th century, the politically, socially and economically marginalised Shia community grew in numbers, something that has not been reflected in the political accommodation (the Maronites have repeatedly blocked a new census) and contributed to the 1975-90 civil war. The rise of Hezbollah is in part the product of Lebanon's entrenched discrimination; it is a system riddled with sectarian triggers.

The origins of the latest crisis lie in the events of Valentine's Day 2005, when the then Sunni prime minister, Rafiq Hariri, was killed along with 22 others after a huge bomb exploded as his motorcade drove through central Beirut. On that day, Lebanon's diffuse political elite congealed into two loose blocs that have faced off against each other ever since.

Hariri's assassination ignited the Cedar Revolution, which led to the formation of the so-called March 14 alliance (the date in 2005 when thousands of Lebanese took to the streets of Beirut to demand an end to Syrian occupation). March 14 is led by Rafiq Hariri's son Saad, who succeeded his father as prime minister, and is a coalition mainly of Sunni and Christian parties. The west's preferred partner, it is backed by Saudi Arabia, where Saad Hariri grew up.

Opposing it is the March 8 alliance. Broadly supported by Iran and Syria, the coalition includes Hezbollah and non-militant Shia and Druze parties, and takes its name from 8 March 2005, date of the first counter-demonstrations against the Cedar Revolution. March 14 and March 8 governed together until their coalition government fell in January last year.

The Hariri assassination created something else, too: the United Nations Special Tribunal for Lebanon. Set up to indict and prosecute Hariri's killers, it has a mixed composition of Lebanese and international judges and an international prosecutor. Predictably, the tribunal divided Lebanon along party (and therefore sectarian) lines. March 14 supports the tribunal; March 8 denounces it as an Israeli-American tool designed to smear Syria and Hezbollah.

Throughout 2010, the country's already toxic political debate was poisoned further by rumours as both sides waited for the indictments to be handed down. Leaks indicated that people in Hezbollah would be named as the killers of Hariri, a charge the party angrily denied. Hasan Nasrallah, who leads Hezbollah, repeatedly attacked the tribunal's integrity and threatened to "cut off the hands" of any "collaborators". In early January last year, indictments were submitted but they remained sealed; fearful of what was coming, Hezbollah demanded that the government end co-operation with the tribunal and reject any findings. Saad Hariri refused, and on 12 January Hezbollah and its allies resigned from the coalition government.

Shortly before the government's collapse, I went to see Fares Soueid, the general secretary of March 14, at his Beirut office to learn more about Rafiq Hariri's murder seven years ago. On the day of the assassination, Soueid was with Hariri in the Lebanese parliament, drinking coffee in the bar outside the debating chamber. Hariri left, and five minutes later Soueid heard the blast. He knew it was a bomb - this is Beirut, after all - but it never crossed his mind that it might be his friend. "For us, Hariri was a superman," he told me. "We thought the guarantees from the United States and the Arab world would keep him safe. He was a 'Muslim with a tie': he wanted to show the world that Islam is not terrorism, and that it can work with the international community." Who killed him? "The Syrians and Iranians, through Hezbollah. The message was simple: 'There is no immunity for any Muslim in the Islamic world who has relations with the west.'"

For Soueid, Hariri's death changed everything. "Before the assassination, the demand [for Syria's withdrawal from Lebanon] came from the Christians only," he told me. "After the assassination, the alliance was Maronite, Druze and Sunni. Only the Shias opposed it."

Unstoppable force

Sami Gemayel, scion of one of Lebanon's most prominent Maronite families, is another important member of March 14. Both his father, Amine, and his uncle Bachir, who was assassinated in 1982, served as president of Lebanon, a post that he, too, is tipped to hold one day. With Garo, my Armenian driver, I drove through the city's Christian quarter to see him.

On the pavements either side of us walked immaculately dressed women wearing dark blue jeans, pashminas and Ray-Bans. We took a corkscrewing road up Mount Lebanon, the Christian stronghold. As we rose, the landscape of civil war Beirut stretched out beneath us. Tangled steel ruins and bombed tenements shimmered in a silvery film of mist. "We don't want this again," Garo said.

The Gemayel residence is a sprawling brick and stone complex where, as president, Amine Gemayel would receive distinguished guests. Shaven-headed militiamen with low-slung AK-47s leaned against 4x4s strategically placed around the entrance. Once inside, I passed through a series of stone courtyards with coffee tables and benches scattered amid cedar groves and thick shrubbery. Sporting Harvard chic, Gemayel wore beige slacks and a V-neck jumper over a white shirt; tied around his wrist was a dangling crucifix. "Hezbollah accepts that the tribunal cannot now be stopped," Gemayel explained. "The strategy is to claim that a western-sponsored tribunal wants to label Hezbollah the murderers of a Sunni leader because they fight Israel. Their goal is not to lose public opinion in the Arab world. The best way to ensure this is to have the Lebanese government say it is all nonsense."

Many fear that if members of Hezbollah are indicted, there will be violence, maybe even war. "Hezbollah are trying to take hold of the government so they don't have to use violence," he said. "But if they are not successful, they will use force and we will have to protect ourselves." How? I asked. He paused. "By any means that we can."

According to Gemayel, violence back in 2008 between supporters of March 14 and those of the Hezbollah-led March 8 marked the beginning of a shift in power away from the former. "In May 2008 Hezbollah attacked [the Druze leader Walid] Jumblatt in his Chouf Mountains stronghold, and this still haunts him. He believes allying with March 8 will best guarantee the Druze's security. The west and March 14 were not able or willing to protect their allies, even politically." With Jumblatt's party on board, March 8 had sufficient numbers to trigger the government's fall in 2011.

A few days later, I got to speak to Jumblatt. Our conversation was pointed. Why had he joined March 8? "I started with March 14 but it was clear during 2005-2008 that, under American pressure, their policies would lead to sectarian strife and civil war," he told me. "Since Hezbollah's defeat of Israel in 2006 the Americans have been totally focused on attacking them - even at the price of Lebanese stability. I refuse this. Washington cares nothing for Lebanon or its future."

And what of the tribunal? "[It] is being used as political tool by the Americans. Look at the leaks coming out of it that appear on CNN and Fox News, all designed to smear Hezbollah. You must go back to the failure of Israel's war in 2006; they will try anything to discredit Hez­bollah. I was a vocal supporter of the tribunal in the beginning. But if it comes to it, I do not want to see my country dragged into a sectarian war. I choose stability over justice."

For the outnumbered Druze, conflict could be fatal. When I asked Jumblatt to explain his alliance with Hezbollah, his response had the quality of a mantra. "I support the Palestinian cause, which Hezbollah fights for," he said; "and I support Hezbollah in its struggle against Israeli aggression.

“It is true that in the past we exchanged some violent words," he added with understatement. In 2007 he labelled Hezbollah a "state within

a state", and his subsequent volte-face is more likely a reflection of the west's inability to protect its Lebanese allies. March 8, it seems, was now united and politically focused.

On a bright and crisp morning, I convinced Garo to take me into Hezbollah-controlled south Beirut to meet "Mohammad", a Hezbollah supporter. After we'd spent an hour inching through traffic, the southern tenements appeared on the horizon. I had crossed an invisible frontier. Litter and rubble replaced Armani and Max Mara. Around us, pictures of Nasrallah and Ayatollah Khamenei of Iran adorned walls and hung from lamp posts. Veiled women hurried by, ubiquitous but anonymous in thick black hijabs. As we pulled up, a group of Palestinian youths lethargically kicking a football around glanced over at our car. “Hurry up," Garo said.

I found Mohammad among piles of fake Levi jeans and Rolex watches behind a makeshift market stall. He told me that the Israelis had killed Hariri and the international tribunal was a Zionist plot to discredit Hezbollah. "We protect Lebanon and resist Israel's war on the Arabs. Now they want to smear us. But it will not succeed. We resisted the Israelis before and we will resist them again." Hezbollah did not want a civil war, he said: in May 2008 it had responded to deliberate provocations by the government and had taken to arms only as a last resort. Hezbollah, he said, would seek to enter a new government and move the country forward; it did not take its orders from Tehran. It was committed to Lebanon's future.

Day of rage

About a week after my meeting with Mohammad, news emerged that March 8 had nominated the Sunni MP Najib Mikati to succeed Saad Hariri as prime minister, and that he would begin consultations for the formation of a new coalition government immediately.

Despite his well-known closeness to Syria, Mikati is a consensus figure, popular with both March 14 and March 8. In 2005, he served three months as prime minister in a caretaker government and impressed all sides with his talent for compromise. But the sectarian obstacle is insuperable. Some said Mikati was Hezbollah's man; and, viewed in this light, a Sunni prime minister was now beholden to a Shia party.

On the day of his appointment, Sunnis across the country held a "day of rage", blocking roads with burning tyres as they vented their anger at Hariri's perceived overthrow.

As the consultations continued, I managed to speak to Saad Hariri's chief political adviser, the former finance minister and ambassador to the US Mohamad Chatah. The two men had been in close contact throughout the crisis as March 14 planned its next move. I asked him if he thought, as many were saying, that Mikati was obligated to Hezbollah. "Hezbollah brought Mikati into the government," he replied. "They appointed him." This left Mikati in a potentially awkward position. "Hezbollah is a legitimate political party," Chatah said, "and they enjoy considerable support, especially among the Shia population. When the country was under Israeli occupation they resisted bravely, and that is admirable. But you cannot ignore the fact that Hezbollah is a military force that operates independently of the Lebanese government, and that pursues an ideology not shared by the majority. More than this, the fact they are in alliances with countries that have agendas separate to Lebanese interests puts Lebanon in danger."

Chatah had recently been at a gathering of Sunni leaders, including Mikati, that reaffirmed a commitment to the UN tribunal. It now remained to be seen whether he would abide by this. "Hezbollah regard him as their choice. Whether he can go beyond that and control the government is the big question that now faces Lebanon."

Chatah told me that March 14 would refuse to join what it perceived to be a Hezbollah government, and he was true to his word. In early March, Saad Hariri officially declined Mikati's invitation to enter a coalition. Despite losing power, March 14 was politically well placed. Hezbollah was trapped. Attempts to neuter the tribunal had failed and the indictments would soon be made public, which left it with two options: accept the findings, which was inconceivable, given that it was likely it would be indicted, or reject them. But without Hariri in the governing coalition it would seem, even to the Arab world, that Syria and Iran were manoeuvring again. Worse, Hezbollah would be labelled killers of a Sunni prime minister in a largely Sunni Middle East. By this chain of reasoning, all Hariri had to do was wait and then obliterate March 8 at the next general election in 2014.

A country, not a nation

But Hariri appeared hamstrung by his allies. Against the backdrop of the Arab spring, the US was so deeply reviled by those Middle Eastern states not allied with Washington that it had become a liability. After a fairly innocuous meeting with a minor Lebanese official, the US ambassador was given a public dressing down for "interfering" in Lebanese politics. Meanwhile, the influence of Saudi Arabia, long Hariri's main regional backer, had receded and an increasingly assertive Turkey had taken the role of mediator. The region, Prime Minister Recep Tayyip Erdogan declared last January, "could not cope with Lebanon entering a new atmosphere of uncertainty".

Sitting on a tree-lined avenue in west Beirut, I spoke to Misbah Ahdab, a Sunni and former March 14 MP from a prominent Tripoli family. Over coffee and Marlboro Lights, he told me: "So much of what happens in Lebanon is linked to what happens in Iraq, in Iran, in south Yemen, in Syria and in Palestine . . . This has always been the case. Syria meddles for regional, historical and political reasons, Saudi Arabia for financial, Israel for regional hegemony and its security fears. We are a small state used as an arena for the battles of others. Everyone has an agenda here. This is my country and I love it, but it is not a nation; it is a country.

“Look," he continued, "Hezbollah and March 8 are in the ascendancy. If it's local, Hezbollah controls it. They control the country. The international community might want some change and pressure for some sort of coalition to be formed. The US could come to a deal with Iran that decapitates Hezbollah. But leaving it as is - Hezbollah wins."

His words proved prescient. Consultations continued over the next few months until, on 13 June, five months after the government fell, March 8 at last announced a new cabinet. Led by Mikati, it gave Hezbollah and its allies 17 out of 30 cabinet seats - up from the ten it had held

in the Hariri coalition. This put a group that the US state department considers a terrorist organisation in control of Lebanon. Iran was seemingly on Israel's border.

Then the indictments came down. On 30 June the tribunal issued four arrest warrants to the Lebanese authorities, and on 17 August it published the names of those indicted: Salim Ayyash, Mustafa Badreddine, Hussein Oneissi and Assad Sabra, all, unsurprisingly, members of Hezbollah. Equally unsurprising was Hasan Nasrallah's refusal to hand the men over; publicly, he doubted they would ever be found.

Yet despite Hezbollah's steadfastness, the Arab spring - which had initially turned events in its favour - has proved problematic. Nasrallah's loud rejoicing as pro-US dictators toppled across the region turned to a more voluble and noticeable silence when the revolution hit Syria. Hezbollah has no choice but to stand by its patron Bashar al-Assad, the Syrian president, and if Syria falls then so does its main source of funding and support. Meanwhile, Hezbollah's other big backer, Iran, faces increasing inter­national isolation over its nuclear programme. Perhaps most importantly, a party that derives legitimacy from fighting oppression cannot sustain its image while supporting a regime that murders its own people for the crime of wanting more freedom. The Israeli flags that once burned in Damascus have, for the time being, been replaced with those of Hezbollah.

In Lebanon, the internal politics is proving equally complicated. For several months the new government was divided over the country's mandatory financial contribution to the UN tribunal. Mikati, Jumblatt and the Lebanese president, Michel Sleiman, favoured paying the required money to avoid confrontation with the UN. Hezbollah opposed making any payment. In reality, the dispute was over the tribunal's legitimacy - to pay would be an implicit acceptance of its authority; to refuse, an outright rejection. Despite Hezbollah's best efforts, this was a battle it lost at the end of November, when payment was made.

Jumblatt may yet prove a vital player once again. In an October interview on al-Manar, Hezbollah's TV station, he emphasised his ties to the party but expressed pointed dissatisfaction over its unconditional support for Damascus. Lebanon's political weathervane may yet turn again in expectation of the Syrian regime's collapse, and the chance to abandon Syria may prove too much of a temptation to resist. Jumblatt has personal as well as political reasons for welcoming the end of the Assads - Middle East lore has it that Hafez al-Assad, Bashar's father, had Jumblatt's father killed in 1977. Shortly after succeeding his father as head of the clan (the Jumblatts, like the Hariris and, indeed, the Assads, are dynastic), Jumblatt was summoned to Damascus to meet Assad, Sr in what must have been an already unpalatable appointment. His attempts to assert the Druze cause were met with a broad smile from Assad, who addressed his young interlocutor in a sweet, paternal tone. "Walid," he said, "sitting there like that, you remind me of your dear father."

Towards the end of the year, I contacted Gemayel again to ask him how things now stood. Hezbollah, he agreed, had been weakened by regional events but March 14 had not capitalised on the opportunity. Saad Hariri remained out of the country, shuttling between France and Saudi Arabia and depriving the coalition of its leader. The governing alliance, he felt, remained resilient as long as Hezbollah was strong. "We just don't know if Mikati will abide by Hezbollah's diktat or if he will end up resigning to safeguard his credibility," he said.

Where, I asked, would things go from here? "I don't know," he replied. "But it is very serious. The Middle East is in a transitional phase, and it is a very difficult one. We are witnessing the demise of old ways of government and the establishment of new ones."

David Patrikarakos tweets @dpatrikarako