Israel appears to be preparing for the next phase of its military operation: a ground campaign to “crush and destroy” Hamas, as Israeli Prime Minister Benjamin Netanyahu has put it.

Israel has signaled that it might be willing to delay an invasion – but not call it off entirely – if Hamas releases more hostages. But that means an invasion is still very likely, which raises questions about how Hamas has prepared for a ground invasion and whether Israel is prepared for what could be a long, drawn-out fight.

Prior ground attacks from Israel into the Gaza Strip have been dangerous, deadly and costly for both sides.

The most recent significant ground campaign, known in Israel as Operation Cast Lead, occurred over a three-week period from December 2008 to January 2009.

According to the Israeli military, that operation was launched to strike the Hamas infrastructure that enabled its terrorist and rocket attacks against Israel. In that battle, thousands of Israeli troops fought Hamas fighters, with an Israeli cease-fire declared on Jan. 17, 2009. According to some accounts, losses in that operation totaled at least 13 Israeli military fatalities, 600 to 700 Hamas deaths and over 1,400 dead Palestinian civilians in Gaza.

Since that conflict, up until the horrific Hamas attacks of Oct. 7, 2023, Israeli operations in Gaza have mostly involved airstrikes against Hamas, hitting targets in the Gaza Strip. In the wake of the Oct. 7 attacks, Israel has stepped up airstrikes, but also massed troops, tanks and other equipment on its border with Gaza.

The international community also expects a ground invasion. Former U.S. President Barack Obama has said an Israeli ground operation could “backfire” if civilians aren’t adequately protected.

Hamas has been guarded about its own details, but says it has prepared, with Iranian support, not only for the Oct. 7 attacks but also to respond to an Israeli ground campaign – including taking action outside the Gaza Strip if there is an invasion.

As a former U.S. government intelligence and counterterrorism senior official, who now teaches about those topics and national security, I expect that once combat begins, fighting will be intense. The conflict will likely resemble heavy urban fighting similar to other battles over the past 20 years elsewhere in the Middle East against Iraqi militants and the Islamic State group – and very different from the more limited engagements Israel has attempted in Gaza up until now.

Combat operations in densely packed urban environments are among the most complex for military planners and the troops who have to fight in them for a variety of reasons. The physical space is dense, with above-ground structures or subterranean networks that provide ample environments for fighters to attack, remain concealed or move without detection. There are narrow channels like alleyways or roads that military units have to navigate through. Large numbers of noncombatant civilians are also around. These factors can complicate the ability of even the best-trained troops to accomplish their objectives while also minimizing their risk.

Nowhere for Hamas to go

Though Israel estimates having killed more than 1,500 fighters during and in the days immediately following the Oct. 7 attacks, its military estimates that Hamas probably has tens of thousands more well-armed fighters in Gaza.

Hamas fighters have nowhere to fall back to in the face of an attack by Israel. The strip’s borders with Israel remain sealed, with only limited openings at the Rafah crossing with Egypt to allow for humanitarian aid to enter. Recently, Cindy McCain, head of the United Nations World Food Program, warned that the continued Israeli blockade around Gaza has pushed the civilian population there into a grave humanitarian crisis. But Egypt has been reluctant to allow people through, citing both humanitarian and foreign policy concerns.

With nowhere to go, it is highly possible that Hamas will decide to stand and fight an Israeli invasion. At that point, Hamas will likely use suicide attackers and the weapons it has and can make – some combination of roadside bombs, booby traps, improvised explosive devices, rocket-propelled grenades, automatic weapons, mortars and snipers.

In addition, Hamas has built an extensive network of as many as 300 miles of underground tunnels throughout Gaza, which its fighters will use to hide and travel in. The Israeli air campaign since Oct. 7 will also help Hamas, because it has destroyed buildings and created piles of rubble that have not yet been removed, making above-ground travel of Israeli forces difficult.

Israel will face further political and humanitarian risks because Hamas kidnapped dozens of hostages on Oct. 7, and their locations are unknown. Even if some are released before an invasion, Israeli attacks could injure or kill any who remain. And rescue operations would require precise intelligence and careful military planning to work in a very small physical area with widespread fighting.

Israeli forces have not faced these conditions often or for very long in the past, but other nations’ militaries have.

The battles of Fallujah

In 2004 and 2005, thousands of U.S. Marines and troops from other nations in an international coalition fought Iraqi insurgents and members of al-Qaida in Iraq in Fallujah, Iraq.

While they inflicted significant losses on those adversaries, U.S. and allied troops also took heavy casualties.

In the first battle of Fallujah in early 2004, 38 U.S. troops were killed and at least 90 injured, with at least 200 al-Qaida or Iraqi insurgents killed and an unknown number of civilians killed or injured. In the second battle of Fallujah, later in 2004, U.S. troops suffered 38 fatalities and 275 injured, with upward of 1,000 to 1,500 insurgents killed and another 1,500 injured. Combined, these were the two biggest urban battles for U.S. forces during the Iraq War.

In addition, much of the city of Fallujah, which once had a population of 250,000, was destroyed, and required significant reconstruction efforts before residents could move back in – only to be displaced again when the Islamic State group emerged and also fought there against the Iraqi government in the mid-2010s.

A decade later, U.S.-backed Syrian Democratic Forces and the Iraqi military took on fighters from the Islamic State group in Iraq and Syria, commonly known as ISIS, in cities like Baghouz and Raaqa, Syria, and Mosul, Iraq. Those fights resulted in tens of thousands of ISIS fighters killed or captured. The survivors, having lost control of any territory, went into hiding.

In these urban ground offensives against ISIS in Iraq and Syria, the losses for the Iraqi military and the Syrian Democratic Forces were heavy, totaling over 1,000 for each of these forces. And just like in the battles in Fallujah, civilian deaths and injuries also occurred in high numbers due to the intensity of the urban combat and its proximity to regular people trying to live their lives.

Lessons for Israel?

In late October 2023, the Pentagon dispatched Marine Lt. Gen. James Glynn and other military advisers to Israel to consult on plans for a ground operation in Gaza.

Glynn fought in Fallujah and advised the Iraqi military in its fight against the Islamic State group in Mosul. He was expected to offer advice based on his experience in protracted urban combat, including ways to minimize civilian casualties.

No one knows precisely how events will unfold in the coming days. If Israel does indeed mount a ground campaign, the resulting fight between the Israeli military and Hamas will almost certainly be violent and difficult.

Casualties on all sides of the conflict will be high, and will include innocent Palestinians who have not left the northern part of Gaza for the southern end of the strip, where humanitarian aid and relief is beginning to arrive. The ensuing urban battles may resemble those in Fallujah in the mid-2000s or ISIS stand-offs a decade ago.