(Reuters) - Yemen’s warring parties have failed to leave the main port city of Hodeidah three months after agreeing to a breakthrough U.N.-led truce deal as part of efforts to end a four-year-old war that has pushed the country to the brink of famine.

The following is a timeline of the impoverished Arabian Peninsula country’s slide into violence and how the conflict has developed.

- 1990. Unification of north and south Yemen to form a single state under president Ali Abdullah Saleh.

- 1994. Civil war in which Saleh prevents south, angered by what it sees as its lower status, from splitting with north.

- 2003-09. Houthi group in north protests marginalisation of the local Zaydi Shi’ite Muslim sect and fights six wars with Saleh’s forces and one with Saudi Arabia.

- 2011. Arab Spring protests undermine Saleh’s rule, lead to splits in the army and allow al Qaeda in the Arabian Peninsula (AQAP) to seize swathes of territory in the east.

- 2012. Saleh steps down in a political transition plan backed by Gulf states. Abd-Rabbu Mansour Hadi becomes interim president and oversees a “national dialogue” to draft a more inclusive, federal constitution.

- 2013. AQAP survives military onslaught and drone strikes, staging attacks across the country while retaining a persistent presence. Saleh and his allies undermine the political transition.

- 2014. The Houthis rapidly advance south from Saadeh and seize Sanaa on September 21 with help from Saleh. They demand a share in power.

- 2015. Hadi tries to announce a new federal constitution opposed by the Iran-aligned Houthis and Saleh, who arrest him. He escapes, pursued by the Houthis, triggering Saudi intervention in March along with a hastily assembled Arab military coalition.

Months later the coalition, aiming to restore Hadi’s ousted government, drives the Houthis and Saleh loyalists from Aden in south Yemen and Marib, northeast of Sanaa, but the front lines solidify, setting up years of stalemate.

- 2016. AQAP takes advantage of the chaos to establish a mini-state around Mukalla in east Yemen, raising fears the war will lead to a new surge in jihadist activity. The UAE backs local forces in a battle that ends the group’s rule there.

Hunger grows as the coalition imposes a partial blockade on Yemen, accusing Iran of smuggling missiles to the Houthis through Hodeidah alongside food imports, something it denies.

Coalition air strikes that kill civilians prompt warnings from rights groups, but Western support for the military campaign continues.

- 2017. The Houthis launch a growing number of missiles deep into Saudi Arabia, including at Riyadh. Seeing a chance to regain power for his family by reneging on his Houthi allies, Saleh switches sides, but is killed trying to escape them.

Friction also develops between fighters in Aden backed by Saudi Arabia, and those backed by its Emirati coalition partners.

- 2018. Coalition-backed forces, including some flying the southern separatist flag, advance up the Red Sea coast against the Houthis, aiming to take the port of Hodeidah. Hodeidah handles the bulk of Yemen’s commercial and aid imports and is critical for feeding the population of 30 million.

Military stalemate ensues: the Houthis control the port and coalition-backed Yemeni forces mass on the outskirts.

In the first major breakthrough in peace efforts, the warring parties agree at December talks in Sweden, the first in two years, on a ceasefire and troop withdrawal from Hodeidah. Work on a prisoner swap also begins.

-2019. The Stockholm Hodeidah truce largely holds but the withdrawal has yet to materialize amid deep mistrust between the Houthi movement, which controls Hodeidah, and the Saudi-led coalition massed on its outskirts. Violence continues in parts of Yemen outside Hodeidah.

For a picture essay on hunger in Yemen, click on reut.rs/2Y7lBtz