Yet a significant difference is that the world has a ringside seat through round-the-clock television coverage and the Internet. That, coupled with the rising advocacy power of human rights groups, heightens the burdens on conventional armies with civilian casualties a near certainty in urban combat.

Israel has been through this type of fight before — entering Lebanon as far as Beirut in 1982 and more recently pushing into West Bank cities in 2002 and battling Hezbollah militiamen in southern Lebanon in 2006.

The Gaza incursion, military experts say, displays some new tactics adapted from past urban battles such as expanded reconnaissance from unmanned drones, more use of explosive-sniffing dogs and adding armor on the underbelly of Israel's Merkava tanks to guard against bomb blasts.

Israel also has been fine-tuning its tactics at an elaborate mock Arab city built on an army base in the Negev Desert. The city has hundreds of structures, including mosques, apartment buildings and a simulated refugee camp. Maj. Avital Leibovich said training drills involve soldiers posing as militants, civilians and even foreign journalists.

But it also includes traditional assaults such as shelling and missile strikes from Apache helicopters that risk civilian casualties.

Israel has reportedly used — for the first time — bunker-buster bombs known as GBU-39, an American-developed weapon with precision targeting systems and a limited blast radius that's intended to reduce the chance for widespread casualties.

Gaza has one fundamental similarity to major urban battlefields since World War II: the Vietnam War's Battle of Hue in 1968; Beirut's civil war in the 1980s; the U.S.-led force in Mogadishu in 1993; Russia's push into breakaway Grozny, the Chechnya capital, in the 1990s; and Fallujah, Najaf and other cities across Iraq.

"It's a nasty kind of war," said Jonathan Fighel, a senior researcher at the International Institute for Counterterrorism in Herzliya, Israel.

He described it as a "jumble of variables" that most military forces seek to avoid.

"It's dynamic, constantly changing," he said. "You have to identify your enemy in the midst of a civilian population — an enemy that is using that population as a human shield. Who is a combatant? Who is a noncombatant?"

There is also the risk of becoming mired for years.

Stephen Graham, a professor at Durham University in Britain, has called it the "black hole" for modern military forces — when militias coordinating on mobile phones and setting remote-detonated bombs can pin down armies that have far superior firepower and technology.

About two hours after Israeli ground troops entered Gaza on Jan. 3, the country's defense minister, Ehud Barak, said on national television: "This will not be easy and it will not be short."