The inquiry into the Marathon Day bombings by the Tsarnaev brothers in Boston on April 15, 2013, which bombing injured more than 260 people, is seeking explanations as to the motives of the perpetrators.  What were the fundamental reasons for the deadly attack?  Why did a 26-year-old Muslim married man with a small child, with no apparent training and no declared affiliation or connection with a terrorist group and aided only by a 19-year-old younger brother, become a brutal and callous murderer? How was the older man, Tamerlan Tsarnaev, who had been a boxer in the United States and had some education before dropping out of college, transformed at some point in 2009 into an Islamic extremist who gave up boxing and drinking?  Why did the younger brother, Dzhokhar, a member of the wrestling squad at school, a pre-medical student at the University of Massachusetts at Dartmouth, and a lifeguard at a Harvard pool, agree to assist Tamerlan?

Several explanations can be proposed.  The members of the Tsarnaev family, especially the father, were said to be Sufi Muslims, relatively moderate in their Muslim faith.  But the mother is another story.  After an uneven history during her years in the United States, which includes accusations of stealing designer dresses worth more than $1,500 from Lord and Taylor, she seems to have become an Islamic extremist and may well have influenced her sons.  An explanation, frequently given in cases of fanatical behavior, of alienation from the society in which the individual is living is not viable, because as late as September 2012, Tamerlan applied for U.S. citizenship.  He appears to have had an intemperate personality, shown by the fact he had been twice evicted from the mosque he attended in Cambridge for belligerent behavior, and warned he would be expelled if he engaged in any future undesirable actions.  The unsuccessful boxer with his disposition to violence, a mental disorder, may psychologically have been seeking revenge on society for his own athletic failure.

Whatever the keys to the personalities and temperaments of the two men and the explanations for their actions, their murderous activity is another warning to democratic countries of the continuing threat from Islamic fundamentalists to the safety and security of their citizens.  The two men were of Chechen descent but had never lived in Chechnya; in fact, they were born in the central Asian republic of Kyrgystan, an area to which thousands of Chechens had been deported by Stalin in 1944.

It is the relationship of Chechen and those of Chechen origin with Russia which is at the heart of the continuing violence and extreme Islamist adherence and activity since the end of the Soviet Union in 1991-2.  Jihadist violence in the North Caucasus has occurred for some time, but its greater magnitude today dates from the war launched in December 1994 by President Boris Yeltsin to force the republic of Chechnya back into the Russian Federation that had replaced the Soviet Union.  Russia had refused to allow the republics within the Federation any meaningful autonomous status.  Before Yeltsin was obliged in August 1966 to withdraw the Russian forces which had met with strong Chechen resistance, that resistance, once mainly nationalist in outlook, had become influenced by foreign Islamist militants.  Among them were Saudi Arabians who had fought against Russia in the war in Afghanistan in the 1980s.  The Chechens, an impoverished people, became receptive to the jihadist Islamic Wahhabi message.  Their plan was to defeat the Russians in the North Caucasus and create an Islamic state there.

Chechen hostility continued with bombings , by individuals called "Chechen bandits" by the Russians, in Russian cities, causing more than 300 deaths.  The consequence was another invasion by Russia to restore order in October 1999.  In a brutal war the Russians defeated the Chechen resistance.  The 1994-6 war had caused the death more than 80,000.  The 1999 war led to the forced flight of more than 200,000 Chechens.  But the price paid by the Russians was continuing Islamic militancy against them, which included two dramatic actions by Chechens.  One was the seizure in October 2002 of the Dubrovka Theater in Moscow when armed Chechens took 850 hostages.  The other was an attack on a school in Beslan in September 2004, when Chechen terrorists took more than 1,100 people as hostages.

After the president of Chechnya was assassinated in 2004, Vladimir Putin nominated Ramzan Kadyrov, the 36-year-old son of the former president, to succeed him.  Kadyrov was installed on March 2, 2007.  He is a devout Muslim anxious to enforce sharia law and the wearing of hijabs and long skirts for women in the country, but he has attempted to keep the republic relatively stable.  By contrast, the anti-Russian rebels, mostly outside the country since 2002, have become fanatical Islamic extremists rather than nationalists.  Their leader is a man named Doku Umarov, head of the terrorist group known as the Caucasus Emirate, whose objective is to create an Islamic state and which operates across the North Caucasus.

Umarov declared himself the first emir of this Emirate on October 31, 2007.  The group has been responsible for more than 2,000 attacks and 47 suicide bombings; in 2012, it claimed 566 attacks in the North Caucasus.  The attacks have included the Moscow subway bombing that killed 40, the attack on Moscow's Domodedovo airport that killed 36, and bombings of trains on the line between Moscow and St. Petersburg.

The nature of the Emirate is well recognized. Both Russia and the U.S. regard the Emirate as a terrorist organization.  On July 29, 2011, a committee of the United Nations Security Council determined that the Emirate was associated with al-Qaeda.  The Chechen Islamists with a jihadist group known as the Islamic International Peacekeeping Brigade had invaded Dagestan as starting point for an Islamic Emirate.

The extremists have organized bands on the border of Chechnya and Dagestan, a Russian republic which is a major hive of Islamist terrorist activity and of demands for independence from Russia.  This activity is the crucial link with Boston.  Tamerlan lived for several months in this area, mainly in the city of Makhachkala, a city on the Caspian Sea, in Dagestan.  He must have been exposed to and clearly absorbed the ethos of the extremists and perhaps was trained there to commit violent acts.  It is relevant that his mother, who became a religious extremist, was an ethnic Avar from Dagestan.

The remaining problem is why the attack on the US rather than limiting all actions to those against the Russians.  One cannot regard the Tsarnaev brothers as sophisticated geopolitical analysts.  At this point, it is still unknown if they were lone actors or were influenced by a persuasive Islamist, as was Major Nidal Malik Hasan, the U.S. Army Medical Corps officer who killed 13 people at Fort Hood on November 5, 2009, who allegedly had links with Anwar al-Awlaki, a senior recruiter for al-Qaeda.  Yet the assault of Tamerlan and his brother can be explained only by the links between the Islamists in the North Caucasus and the Taliban and al-Qaeda, and the general threat to the United States and to the West of religious extremism.

The Boston Marathon tragedy has reminded the world of the global nature of the terrorist problem.  Globalization and the instant communication made possible through the internet, social networking, and cell phones contribute to a vital and progressive society, but they also help to create the network of known and covert terrorist groups, and those sympathetic to terrorist organizations.  The United States now has to recognize that the danger from Islamic terrorism to Western democracies and to non-Islamist Muslim societies emanates not only from the Taliban and al-Qaeda and is not confined to countries like Afghanistan and Yemen.  It is now widespread throughout the world.