From the mouth of the Rio Grande at Brownsville on the Texas Gulf, the Mexican Border runs a thousand miles up river to El Paso—“The Pass”—then west for an equal length along five arrow-straight survey lines that slice unbending through the Sonoran desert, across the Continental Divide and a dozen lesser ranges, down to the surf at San Diego.  For much of its vastness, the border is guarded only by the elements—searing heat, howling winds, scorpions and rattlesnakes. But in those places where humans concentrate, the ebb and flow is funneled through two dozen ports of entry ranging from a hand-powered ferry across the Rio Grande at Los Ebaños, to twenty-four lanes of traffic pouring north from Tijuana into San Ysidro, California.

Supervisory Customs Inspector Tom Isbell can look out the broad windows of his office and see the whole sweep of the line at San Ysidro, two dozen men under his command searching the river of vehicles with rapid eyes, waving the drivers through with a flick of the wrist, stopping one here and there with a raised