The equation is a grimly simple one: Global warming is already setting off mass migrations involving tens of millions of people around the world.

The prosperous industrialized nations are already full of multiple millions of illegal immigrants understandably seeking a better life than the harsh, grinding poverty and increasingly unbearable changing weather conditions they are suffering in poor nations of Latin America, Africa, the Middle East and the Asian subcontinent.

However, the global economic crisis is making jobs increasingly hard to find for the native populations of the major industrialized nations. Already some countries, such as Russia, are trying to actively send large or significant numbers of immigrant workers back to their nations of origin -- in the case of Russia, usually from the former Soviet republics of Central Asia. And already some major nations have started beefing up their border defenses.

In the United States, both Republicans and Democrats for decades opposed toughening border security with Mexico. Yet it was President George W. Bush, a Republican conservative president who came to office expecting to prioritize creating closer ties with Mexico, who was forced to start building a limited border fence along the Rio Grande.

Hispanic-Americans voted overwhelmingly for President Barack Obama in the U.S. presidential election last November. However, Obama appointed as his first secretary of homeland security the immensely capable and effective governor of Arizona, Janet Napolitano, who has been at the center of the illegal immigration issue, seeking simultaneously to improve border controls and border security while bringing large numbers of illegals within the legal framework of the United States.

Eventually, the issue of mass migration triggered by the major global environmental changes that are already occurring is bound to require vast increases in both police and military personnel in the more prosperous major industrialized nations.

The history of war, the current experience of mankind and the application of old-fashioned common sense all teach the lesson that if enormous numbers of immigrants can pour into a nation in an unregulated and unrestricted manner, the existing government will need to produce large numbers of police on the ground to provide security and a safe living environment both for them and for the already established community.

Ultimately, securing borders in order to regulate vast influxes of immigrants requires large numbers of loyal, trained and armed soldiers and border guards to do the job. High-tech fences cannot guarantee security without large numbers of soldiers to man them.

That is because human ingenuity, usually fueled by profit or desperation or both, is boundless. Fences left to themselves will always be cut and electrical currents disrupted. The success of Israel's much criticized security fence -- only 5 percent of it is any kind of wall -- that virtually eliminated the suicide bomber onslaught of the Second Palestinian Intifada can be attributed to the fence being backed up by continual army patrols.

The Indian army has had the same experience in making its own border fence along the Line of Control in Kashmir to prevent mujahedin guerrillas from entering India across that previously porous frontier with Pakistan.

The standard of Indian special forces in confronting actual terrorist attacks is not yet very good, as the slow and bungled response to the November Islamist attacks in Mumbai showed. But the security fence along the Line of Control has slashed terror attacks and incursions in the state of Jammu & Kashmir by 90 percent, Indian officials say.

(Part 3: Why armies can't be small anymore)