We know the dates of hot wars, 1914-18 and 1939-45, are the best-known examples, but when did the Cold War begin?
There's a theory that the Americans use of the nuclear bomb at Hiroshima in 1945 was the first shot fired in the Cold War - a warning to the USSR that the post-war world would be dominated by the United States.
Perhaps the idea of it was intellectually formalized with Winston Churchill's 1946 'Iron Curtain' speech? Or possibly the following year when the Soviet Union refused to co-operate with the Marshall Plan and responded with the Zhdanov Doctrine stating that the USA was bent on world domination.
This writer believes we are now well into the New Cold War. If that is how it is to be seen, then historians will ask when it began. The answers will be as difficult to pin down as those of the old version.
Before each of the world wars there was an arms race and we are now seeing something similar albeit on a much smaller scale.
Wars do not emerge from nowhere, there are always numerous incidents which, with hindsight, culminate in the outbreak of fighting – the 1938 annexation of the Sudetenland by Hitler being a case in point.
This New Cold War has also had numerous incidents which have led to the current standoff between Russia and the West. Each side has played its part.
After the fall of the Berlin Wall, Russia's economic and military low point was in the 1990s. It culminated in Moscow's inability to come to the aid of its Serbian ally during NATO's war in Kosovo.
From a position of weakness, President Yeltsin had attempted to forge cordial relationships at various levels with the NATO powers and other Western powers.
President Putin's idea upon taking over from Yeltsin was to continue that policy - but from a growing position of strength. What we don't know is if he always intended to become belligerent once Moscow had recovered from the disastrous blow of losing its empire – the USSR.
He made it clear he wanted Russia inside the World Trade Organization, he gave support to the USA after 9/11, and he invited the NATO Secretary-General to Moscow.
In 2003 he became the first Russian leader since Victorian times to be invited on a state visit to the UK, and at a summit in Slovenia he and President Bush got on well. The whole tone of dialogue indicated that a difficult relationship could be managed in a cordial manner leading to closer ties. Conflict did not seem an option.
However, all the time he was busy modernizing the Russian military, even as NATO countries were cutting their defence budgets due to the 'peace dividend'. Putin also never let go of the idea that Russia's 'Near Abroad', the former Soviet states, was Russia's sphere of influence and that the western country had no right meddling there.
This was a clear conflict between the Western countries' support for the former Soviet states to determine their own futures, and Russia's absolute belief that it had the right to shape the region.
From late 2003, the hardliners in the Kremlin were in the ascendancy, determined to restore Russian position as a great power. Things began to change.
In 2004 the Baltics, along with Bulgaria, Romania, Slovakia and Slovenia joined NATO. Putin made his unease clear privately but refrained from overt public criticism.
But the language slowly became harder. The trade and military meetings began to become tenser. The atmosphere became less warm, veering towards cool. Simultaneously the oil price was rocketing. Russia's revenues from gas and oil more than quadrupled in the first decade of the century which funded its massively expanding military budget.
There had already been setbacks in the relationship, for example, the Bush administration's withdrawal from the Anti-Ballistic Missile (ABM) Treaty in June 2002 and subsequent plans to station a missile system in Europe, but whereas before the relationship survived such bumps along the way, now the direction of travel was towards antagonism.
There are numerous examples of the tensions, the 'Colour Revolutions', the murder of Litvinenko in London, Moldova's choice to look west, and Western recognition of an independent Kosovo among many others.
By 2008 Putin felt strong enough to make a speech at the Munich Security Conference in which he said that "The United States has overstepped its national boundaries in every way".
Then came the war against Georgia in August 2008. Georgia's President Shaaskavilli made some disastrous decisions. The campaign to join NATO is one thing, but giving into Russian provocation and launching military action simply gave Putin the opportunity he wanted.
Russia brushed aside the Georgian military and in changing borders by creating an 'independent' South Ossetia and Abkhazia, served notice that any ideas of a cordial relationship with Moscow were over. This should have been the warning that the idea of a peace dividend was dead.
In 2012 Putin ignored Hillary Clinton's clumsy attempts to 'reset' relations and boycotted the Washington G8 summit. Then last year any lingering doubts that Russia had not lurched back towards the bad old days must surely have removed with its invasion and annexation of Crimea.
Moscow may have partially lost Ukraine, but it has gained territory, and in Eastern Ukraine, created a conflict it can heat up or cool down at a time of its choosing. This month it has become increasingly hot, threatening to boil over.
NATO powers have reacted in various ways, the UK Defence Secretary Michael Fallon signalled that Britain may send additional military trainers to Ukraine to support the 75 already there.
The West has responded to this New Cold War. On a regular basis, NATO warplanes scramble to meet their Russian counterparts who are probing NATO defences, and naval patrols have increased in the North and Baltic seas.
The size of NATO's rapid reaction force is to be tripled, bases are being built in the former Eastern bloc countries (Romania for example) and the Americans will 'forward deploy' a brigade's worth of heavy equipment in Eastern Europe. This is a move Russia says may break the agreement between NATO and Russia when NATO first began to enlarge after 1989.
The Russians are boosting their forces in the west of Russia, beefing up military co-operation with Belarus, and embarking on the modernization of its Arctic brigade. Both sides are increasing the number of exercises close to the Russia border, and a major NATO show of force can be expected later this year.
Perhaps most worryingly Russia has announced it will add 40 more intercontinental ballistic missiles to its arsenal amid repeated reminders from Putin about Moscow's nuclear capability.
The West and the Russians are clashing diplomatically elsewhere. Russia and China have conducted joint military exercises in the Mediterranean, and Moscow, having opposed the Libyan intervention, supports the Iranian and Syrian regimes.
This is the backdrop to the New Cold War at a time when channels of communication between NATO and Russia are severely diminished, and each side is glowering at the other across the flatlands of the North European Plain whilst arming them themselves to the teeth.
The Americans believe the situation will not change so long as Putin is in power. He may be in charge until 2024.
When did this New Cold War begin and why? You could argue it different ways. The 'why' is clear – the fundamental differences, and interests the players have in Eastern Europe. When? If you had to put a date on it then the day it became clear was when the Russian troops smashed their way into Ukrainian military bases in Ukraine and annexed Crimea. Mar 22nd, 2014.
His latest book 'Prisoners of Geography: Ten maps that tell you everything you need to know about global politics' can be bought here.