The Chinese military general and strategist, Sun Tzu, wrote nearly 3,000 years ago: "Know yourself. Know your enemy."
But certain reaction here to the spread of Isil-managed savagery in Europe shows that some people know little about the enemy and not much for sure about themselves.
Arising from that, it is evident that the West has become a soft touch, too soft for this denouement in what has been described as the clash of civilisations.
As was evident in Paris last weekend, there is no place for such weakness in the heart of Islamic fundamentalism.
If you want to know the enemy, read the works of Sayyid Qutub and Abu Bakr Naji. Qutub was a philosopher of the Islamic revolution, and Naji the strategist of the politico-military doctrine of Isil.
Born in an Egyptian village in 1906, Qutub received a scholarship to study the American educational system. He spent two years in Colorado and California, but travelled widely and was appalled by what he saw. In his eyes, the US was a cesspool of wasteful consumption, exaggerated sex and crass materialism.
Back then, he argued Muslims had to reinstate the pattern and practices announced by Muhammad in the 7th Century. That is, Muslims had to go back to the original pattern, Muhammad's community, to correct such excesses.
His view informed the Isil statement of responsibility for the Paris savagery last weekend. That statement referred to such a cultured city as a "capital of prostitution and obscenity" and the Bataclan concert as a "profligate prostitution party."
Qutub writings have attracted a mass readership, said to be tens of millions throughout the Islamic world, like the various and competing militant groups in Syria, as well as leaders in Saudi Arabia and Iran.
His idea was that Islam was under attack and therefore must defend itself, because failure to do so would be to contravene the intention of God.
There is no doubt, as the conservative US political scientist Samuel P Huntington has written in the Clash of Civilizations in 1992, that "Islam has bloody borders".
In fact, conflict along the fault line between the Western and Islamic civilisations has been going on for almost 1,300 years.
Here is a potted history: After the founding of Islam, the Arab and Moorish surge west ended in 732; from the 11th and 13th centuries, the Crusaders attempted, with temporary success, to bring Christianity and Christian rule to the Holy Land; from the 14th to 17th century, the Ottoman Turks reversed the balance, extended their reach over the Middle East and the Balkans, captured Constantinople, and twice laid siege to Vienna; but in the 19th and early 20th centuries, Ottoman power declined and Britain, France and Italy established Western control over most of North Africa and the Middle East.
After World War Two, the West began to retreat and colonial empires began to disappear - first Arab nationalism, then Islamic fundamentalism, manifested. The West became dependant on the Persian Gulf for energy and, as a consequence, oil-rich Muslim countries became money-rich.
Several wars occurred between Arabs and Israel (created by the West); France fought a ruthless war in Algeria throughout the 1950s; British and French forces invaded Egypt in 1956; American forces went into Lebanon in 1958; subsequently, American forces went into Lebanon, attacked Libya and engaged in various military encounters with Iran; Islamic terrorists, supported by at least three Middle East governments, bombed Western planes and installations and seized Western hostages.
This warfare between Arabs and the West culminated in two wars either side of September 11, 2001, when the World Trade Centre in New York fell.
Abu Bakr Naji has written in Management of Savagery that the attack on the World Trade Centre was "premature". The Management of Savagery, widely regarded to be the Isil playbook, sets the stages towards what would be effectively the Islam Empire.
The first stage - management - is "vexation" of the enemy to create chaos and leave the forces of foreign powers and local proxies distracted and exhausted.
The second stage - the spread of savagery - is essentially what happened in Paris; the third phase - the administration of savagery - is what is occurring in Iraq and Syria, which involves the provocation of all out war with the West.
This will ultimately involve "establishing a fighting society" with means of self-defence; the creation of an intelligence agency and a socio-political programme aimed at "uniting the hearts of the people" by means of money, food and medical services, and the provision of a functioning system of justice under Sharia law.
From this base, it will become "possible to expand and attack the enemies in order to repel them, plunder their money, and place them in a constant state of apprehension and desire for reconciliation".
The immediate tactic, however, is to diversify and widen vexation strikes throughout the Islamic world "and even outside of it, if possible" to disperse the efforts and economically drain the alliance of the enemy, or the West.
In this regard, Naji has cited attacks on tourist resorts, on oil interests and on apostate authors. The tactic has so far proven successful: on its own, the Iraq war is estimated to have cost the United States $1.1 trillion.
Naji has also written: "One who previously engaged in jihad knows that it is naught but violence, crudeness, terrorism, frightening, and massacring..."
The beginning stage was fundamental: "It must be conducted ruthlessly. So must the other stages be affected, since jihad cannot be carried out with softness."
According to the Isil playbook, the ingredient of softness is one of the ingredients of failure for any jihadi action.
After decades, indeed centuries, in the ascendancy, the West has become weakened.
The recent era of what we might call wasteful consumption and crass materialism, and the outlay of billions in wars, has caused Islamic fundamentalists to now seize the moment. The old fault line has been breached and Isil has extended the reach of a new one into the heart of Europe.
This clash of civilisations will continue for years to come, a mere blink of an eye in terms of global history. The people of Europe will have to become accustomed to events such as the savagery that occurred in Paris. Authorities in the West will also have to devise greater security to protect their citizens.
Life, as we know it, has been changing for decades - Paris merely represents a landmark in that change.
In a timeline of 1,300 years, we may be approaching a denouement, but there is no certainty an accommodation will be reached, let alone that a softened West will prevail.
For now, though, citizens in liberal democracies such as ours will need to know when asked: which side are you on?