TEL AVIV // Ever since Israel fought against Arab states, including Syria, in the 1973 Arab-Israeli War, its eastern border with Syria and Jordan has been quietest.
On its northern frontier, Israel battled in 2006 for a month with the Lebanese group Hizbollah, which continues to expand its arsenal in preparation for the possibility of further hostilities.
In the south, Israel has launched two large attacks on the Hamas-ruled Gaza Strip in the past four years in response to frequent rocket fire from the territory.
Also in the south, Israel is finishing a razor-wire fence along its border with Egypt to keep out militant groups on the Sinai Peninsula that have grown in strength since Hosni Mubarak's regime was topped in 2011.
Now the border with Syria - still officially considered an enemy - has returned as a focus of Israel's security strategy amid fears that the civil war there will spill into its territory.
In response, Israel is modernising its fence along the shared border, bolstering cooperation with Jordan - with which it has had a peace pact since 1994 - and rebuilding ties with its once-close ally Turkey.
"Israel is concerned that Syria will become a failed state with a weak central government and no real control over its territory, and that this vacuum will be utilised by non-state actors like jihadist groups or Palestinian groups that will use Syrian territory as a platform against Israel," said Shlomo Brom, a retired Israeli general and a senior researcher at the Institute for National Security Studies, a think tank based in Tel Aviv.
He added that Israel may eventually boost security along its 335-kilometre frontier with Jordan - which includes the occupied West Bank - where protesters have demanded political reform but do not yet pose a serious threat to the US-backed monarch.
Mr Brom said the situation has "escalated dramatically" along Israel's 80km border with Syria, a largely verdant frontier stretching from Mount Hermon in the north to the border junction with Jordan in the south.
Mortar shells from Syria have fallen in the past few months in Israeli-controlled territory and some incidents have drawn Israeli fire at its neighbour for the first time since the 1973 war.
In January, Israel conducted a rare air strike inside Syria near the Lebanese border, hitting a convoy of vehicles with arms it suspected were bound for Hizbollah.
Furthermore, Israel is also concerned that Hizbollah or Al Qaeda could take control of Syria's arsenal of chemical weapons.
Worries have grown as Syrian rebel groups have overrun several towns in recent weeks near the Israeli-occupied Golan Heights, which Israel captured from Syria in the 1967 Arab-Israeli war and later annexed.
"The problem now is how to deal with small groups that may try to shoot at Israeli patrols," said Eyal Zisser, an expert on Syria at Israel's Tel Aviv University. "Israel needs to increase monitoring and intelligence forces and better prepare its soldiers."
In January, Mr Netanyahu said Israel would fortify its security fence along the border with Syria, building a five-metre-high, steel-and-barbed-wire fence with advanced surveillance equipment, early warning systems, trenches and quick-response units.
It was a bid to guard against an influx of refugees or rebel fighters. He said the barrier would be similar to the 230km fence that Israel constructed along its border with Egypt to keep out militants and African migrants seeking to enter the country from the Sinai desert.
Israel has replaced the army division of reservist troops posted at the border with active-duty soldiers that have been better trained to handle potential threats, said analysts.
The unrest has strengthened Israeli and Jordanian cooperation. The Israeli prime minister, Benjamin Netanyahu, has travelled several times to Jordan in recent months to meet King Abdullah, and Israel has asked twice in the past two months for Jordan's agreement that it bomb Syrian chemical weapons sites, the The Atlantic magazine reported in December.
While Israel does not need Jordan's permission, it was concerned that Jordan may be affected by a strike because several potential targets are close to Syria's border with Jordan, which said the "time was not right," the magazine said.
The civil war in Syria has also shifted Israel's diplomatic strategy. Last month, following the first visit in Israel by Barack Obama, the US president, Israel formally apologised to Turkey for the killings of nine pro-Palestinian activists in a botched commando attack on a Gaza-bound Turkish ship, the Mavi Marmara, three years ago.
Mr Netanyahu wrote on Facebook that the unrest in Syria was a "prime consideration" for the apology - which followed three years of cooled relations between Israel and its once-close regional ally.
He added that because both Israel and Turkey share a border with Syria, it was "important" for the two countries to consistently communicate.
Mr Brom said such strategy shifts reflect the potential for increasing violence along the Israel-Syria border.
"In the longer run there could be a real escalation in which projectiles land in Israel every day and very radical Jihadists are dwelling on our border," he said.
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