(Reuters) – An Israeli invasion of Gaza will face an enemy that has built a formidable armoury with Iran’s help, dug a vast tunnel network to evade attackers and has shown in past ground wars it can exact a heavier toll on Israeli troops each time.

As Israel masses tanks on Gaza’s border and suggests that the start of an invasion is only a matter of time, Israel’s generals will look to lessons learned from ground offensives in 2008 and 2014 that also aimed to smash the Palestinian Islamist group Hamas, an Israeli security source and experts said.

In the strongest indication yet that an incursion is imminent, Israel’s military on Friday told civilians of Gaza City, more than 1 million people, to head south within 24 hours.

Yet Hamas has till now, after each past incursion, emerged a tougher opponent. On Oct. 7, it staged its most devastating display when its fighters killed more than 1,300 Israelis – most of them civilians – in the deadliest attack on Israeli soil since its founding in 1948.

Israeli air strikes in response have killed more than 1,400 people in Gaza so far.

Neither side can know exactly what to expect in a ground war. Israel has promised a campaign exceeding anything ever launched to wipe Hamas “off the face of the earth”, while Hamas has shown its ability to survive and surprise, and will be fighting in a dense urban setting with powerful weapons.

“A ground incursion will be like diving into the unknown by the occupation and its forces,” an official in one of Gaza’s Palestinian militant factions told Reuters, giving only his nickname of Abu Abdallah.

Israeli officials have made clear it will not be a swift or easy campaign. And this time it faces the problem that scores of Israeli hostages seized by Hamas on Oct. 7 could now be held in the warren of tunnels that Israeli troops call the “Gaza Metro” and which soldiers must clear to destroy Hamas.

“The purpose is to defeat the entire military capabilities and military apparatus of Hamas and that will demand a long operation,” an Israeli security source told Reuters.

“Most of the targets, people, equipment, logistics are located underground and it’s possible the hostages are located underground,” the source said. “The purpose will be to flatten the ground to then be able to get to the underground bunkers.”

In Gaza, a strip of land with 2.3 million people, the risk of a mounting civilian death toll could also complicate Israel’s military plans.

LEVELLING THE LAND

Based on past experience, Israel’s bunker buster bombs and hi-tech Merkava tanks will be up against booby-trapped tunnels, fighters using the underground network to strike and vanish, and a range of Hamas weaponry that includes Russian-made Kornet anti-tank missiles that Israel first reported used in 2010.

Hundreds of thousands of reservists are being mobilised by Israel for combat against fighters who one former official in Israel’s Shin Bet security service said could number 20,000.

Hamas spokespeople have declined to comment to Reuters about any military operations or planning.

Saleh Al-Arouri, deputy chief of Hamas politburo, told Al Jazeera that before Hamas unleashed its assault on Israel it had a defence plan that was stronger than its attack plan.

Israeli officials have not discussed any military details. A Foreign Ministry spokesperson said: “We are preparing for a ground offensive” and the goal was to heavily damage “terrorist infrastructure” so it was no longer a threat to Israel.

But Gaza residents have already spotted what they believe to be invasion preparations in the intense Israeli bombardment of the enclave since the initial assault by Hamas. Residents say bombing has levelled corridors of land along the border that could open the way for ground forces.

But Palestinian militant official Abu Abdallah said Gaza’s fighters were ready.

“Planes do not determine a battle. Many fighters would prefer to face tanks and forces on the ground, armed by the developed capabilities built up over years,” he said.

Since 2007, when Hamas took control of Gaza from the West Bank-based Palestinian Authority, Israel has launched two major land incursions. Each time it suffered a higher death toll of soldiers – nine killed in 2008 and 66 in 2014.

This time, Hamas has had longer to prepare, making no secret of support coming from Iran, Israel’s arch regional foe.

LEARNING LESSONS

More recently than its Gaza incursions, Israeli troops were confronted with the growing capability of militants in a West Bank clash in June. In a raid on Jenin refugee camp, a militant hotbed, Israeli soldiers were ambushed and pinned down. Apache helicopter support was called in to help.

“The difference between what is happening in Gaza compared to the battle of Jenin camp is between heaven and earth,” said a member of the Jenin Brigade. “We do not have military barracks or heavy weapons such as rockets and Kornets.”

A former senior Israeli commander, who asked not to be identified, said confrontations like Jenin still offered lessons as the military prepared for a Gaza incursion.

“You will have to look at Gaza (as having) much more high capacities and with highly developed technology all over,” he said.

Israel’s government, which under Prime Minister Netanyahu had been one of the most hard-right in its history but now includes opposition figures to build national unity, has said the public must prepare for a long conflict to reach its goal.

“We started the offensive from the air, later on we will also come from the ground,” Defence Minister Yoav Gallant had told soldiers near the border fence on Tuesday.

Shalom Ben Hanan, a former top official in Israel’s Shin Bet security service and now a fellow at the International Institute for Counter-Terrorism at Israel’s Reichman University, said a ground offensive was the only way to dismantle Hamas.

“There is no sterile war. We will suffer casualties,” he said. “We are going to fight, to make combat with very experienced and trained fighters.”

“It must be a decisive win.”