Leon Trotsky famously said even if you are not interested in war, war is interested in you.

The overnight attacks in Paris that's taken more than 100 lives and counting is testament to the truth of Trotsky's words.

Ever since the terrorist strike on Mumbai seven years ago this month, which took the lives of 166 people including a Singaporean lawyer, security experts have braced for the next big strike on a metropolitan city. The issue was not whether there would be another such attack but when. Now, they have it.

Even as the death toll in the Paris strike climbs past 120, it is noteworthy that five of the suspected terrorist attackers of the French capital have been swiftly killed, including at least three in the Bataclan concert hall which was stormed by crack police squads. So too another terrorist, who was nailed at France's national stadium.

These counter-moves suggest French police were prepared for such an outrage. Over the next few days, perhaps even hours, the world will also learn whether they had advance inkling of the impending terror strikes.

Between preserving the freedom of movement of ordinary people - especially a people who value their liberties as much as the French do - and ensuring their safety is no easy task. This was the lesson of the Charlie Hebdo-related murders.

Nevertheless, the authorities responsible for homeland security have done their best by installing closed circuit television in crowded areas and if necessary, infiltrating crowds to keep an eye on suspicious movements.

But how to stop every tourist sporting a backpack? That is the dilemma. Big metros, especially one with a swinging night life, are not easy to defend. The assassins know darkness is their friend and the more crowded the place the bigger the likely impact of their actions. The world saw this in Bali in 2002 and in Mumbai, six years later.

As always when innocents are targeted, the world gropes for a plausible explanation for such outrages.

There is talk that one of the attackers mentioned Syria, where the global coalition is targeting the Islamic State. Others link the strike on Paris to the death on Thursday of "Jihadi John" - as Mohammed Emwazi, the British citizen who was Islamic State's most savage and cruel executioner, was nicknamed.

It is unlikely that an attack of these dimensions could have been organised in so short a time. This is why Trotsky was right - sometimes, war just comes looking for you.