The Litani river runs for around 170 kilometres across most of southern Lebanon. It's situated around 30 kilometers from the border with Israel. It is key for agriculture, irrigating thousands of hectares of farmland, and is a water source for around a fifth of Lebanon's population. 

But it is also at the heart of the current fighting between Israel and Hezbollah, lying at the northern edge of what was once hoped would be a buffer zone aimed at keeping a permanent ceasefire.

In 2000, after over 20 years of occupation, Israel said it would withdraw its forces from southern Lebanon back to something called a United Nations-designated blue line. The blue line is not a border. The UN calls it a temporary line of withdrawal, monitored by its peacekeeping mission known as UNIFIL.

In 2006 there was a 34-day war between Israel and Hezbollah. The UN Security Council unanimously voted for a resolution to end the conflict and pave the way to long-term security.

It's known as Resolution 1701. It called for a full cessation of hostilities and the establishment of a demilitarized zone between the UN Blue Line and the Litani River. Only the Lebanese Army - not Hezbollah - and UNIFIL would be able to possess weapons and military equipment in the area.

The UNIFIL peacekeepers were first brought in during 1978 but after the 2006 hostilities their presence significantly increased. Currently there are around 10,000 troops from 50 different countries: Indonesia has the most, while China supplies around 400 troops.

Any crossing of the UN blue line by land or air constitutes a violation of resolution 1701. But the problem is the key terms of the resolution have never fully been implemented and over the last year there's been almost daily cross-border exchanges.

Hezbollah has maintained a strong presence in southern Lebanon. Its arsenal of rockets and missiles is said to be at around 130,000 - making it one of the most heavily armed non-state groups in the world.

Israel, meanwhile, has been accused by Lebanon of committing 35,000 violations and has now begun a ground incursion into Lebanon.

Israel is demanding Hezbollah withdraw to north of the Litani river - as per the 2006 resolution - so it can ensure the return of around 60,000 residents to their homes in northern Israel.

And as fears grow about an all-out war, Lebanon's Prime Minister Najib Mikati has indicated he is ready to fully implement resolution 1701, which aims to disarm Hezbollah south of the Litani river and deploy the Lebanese army to the area. 

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