For some security analysts, the recent bloodbath in Nairobi's premier shopping mall, in the heart of the Kenyan capital, underlines how terrorists are targeting major cities across Asia and Africa, just as an earlier generation wreaked havoc in Europe's major cities in the 1960s and 70s.

The world cities "are the battlegrounds of the future," says veteran counter-insurgency specialist David Kilcullen, who showed U.S. Gen. David Petraeus how to hammer al-Qaida in Iraq a few years ago.

His strategy effectively crushed al-Qaida, but the subsequent withdrawal of U.S. military forces that ended in December 2011 allowed them to flourish again, killing thousands in their current offensive that now also encompasses war-torn Syria next door.

Kilcullen, a former Australian army colonel, says the carnage inflicted by well-trained, well-motivated killer squads, carrying out meticulously planned and executed operations, as happened in Nairobi Sept. 21-25 and in Mumbai, India's financial capital, in November 2008, is where the world's headed.

Attacks like the high-casualty Nairobi operation by the al-Shabaab group in neighboring Somalia, which is linked to al-Qaida, "reinforce what we've already seen in places like Mumbai and Karachi: That urban environments, including complex pieces of urban terrain like shopping centers, hotels and industrial facilities, are the battlegrounds of the future," Kilcullen observed.

"And the urban siege, with its commando-style tactics and guerrilla infiltration of the big city's ebb and flow, is increasingly the tactics of choice for a wide range of adversaries," he noted in an analysis published by The Guardian daily of London.

Amid the alarming urbanization of the world's burgeoning population, driven by climate change and dwindling resources, cities are swelling out of control.

They're spawning outlaw cultures with access to electronic systems for planning and surveillance, and with the increasing strains this imposes on security, the dangers are palpably growing.

Kenyan authorities say 72 people were killed in the four-day siege of the upscale Westgate Mall, 61 civilians, six security personnel and five attackers.

Some 30 bodies reportedly remain under the rubble of three collapsed floors.

The 12-15 attackers deliberately selected non-Muslims among the hundreds of people in the mall, identifying them by questioning captives on the Koran.

In Mumbai, a megacity of 20 million, 10 Islamist attackers landed by sea from Pakistan and in 12 gun and bomb attacks, coordinated by controllers in a command center using Skype, cellphones and satellite phones, killed 164 people and wounded 308 in a 60-hour rampage.

Nine attackers were killed. The sole survivor was hanged Nov. 21, 2012.

Over the decades, there's been a progression in the ferocity of terrorist bombings, starting with attacks in the 1970s-80s in which fatalities were consciously limited by the perpetrators.

Then came a new phenomenon: Hezbollah's suicide bombings in Lebanon in the early 1980s. The killing threshold of the human bombs rose sharply with deaths by the score or even hundreds. A psychological barrier was crossed.

In the worst atrocity, simultaneous suicide truck bomb attacks on a U.S. Marine barracks and a French paratrooper base near Beirut airport Oct. 23, 1983, killed 299 men in a couple of minutes.

Then there was Osama bin Laden and Sept. 11, 2001, and another threshold passed, beyond anything known before -- nearly 3,000 people slaughtered in airborne suicide attacks.

This was a tactic first plotted by Algerian Islamists in 1994. On Christmas Eve in Algiers, four gunmen hijacked an Air France A300 airbus carrying 232 passengers and crew and planned to crash it into the Eiffel Tower in Paris.

French commandos stormed the jetliner in Marseille during a refueling stop and killed the gunmen.

The security surrounding air travel is so tight these days that such attacks are immensely more difficult to mount.

But low-tech operations in densely populated urban environments, such as Mumbai and Nairobi, are demonstrably easier and new thresholds beckon.

"The Mumbai attack showed that a non-state armed group can carry out ... the type of operation traditionally associated with high-tier special operations forces," Kilcullin noted.

"Indeed, Mumbai was a further demonstration of a long-standing trend, sometimes called the democratization of technology, in which non-state armed groups are fielding highly lethal capabilities that were once the preserve of nation-states...

"Groups like al-Shabaab and al-Qaida will be with us for the foreseeable future, and attacks like Nairobi and Mumbai will remain part of the picture."