Nobody imagined when Congress created the Department of Homeland Security that the department itself would mount the next attack on American transportation, travel and trade. But the department begins an assault this week that will do billions of dollars in damage if it is not stopped.

Starting Thursday, Homeland Security will require U.S. and Canadian citizens to present a government-issued photo ID along with proof of citizenship, such as a birth certificate or naturalization certificate, when they enter the country. Clamping down on border crossings, such as the Ambassador Bridge and Detroit-Windsor tunnel, is motivated by a protective impulse, but its costs outweigh its small security benefits.

In 2004, Congress created the Western Hemisphere Travel Initiative, requiring detailed document checks of every traveler into the United States. A year ago, the Homeland Security Department began to implement the initiative, requiring passports from citizens entering by air from Canada, Mexico and Bermuda. Bedlam ensued.

The State Department was quickly backlogged with nearly 3 million applications for passports. Facing delays of three months, Americans missed business travel and vacations. Even those who paid for expedited processing didn’t receive their passports on time, and they flooded federal lawmakers’ offices with complaints. The three-year cost of issuing additional passports came to $944 million.

Congress got the message. It recently delayed further implementation of the initiative. But Secretary Michael Chertoff announced earlier this month that paperwork requirements at the border would be ramped up. More than 60 million border crossings this year will bear new paperwork requirements. Delays will run many hours. Cars and trucks will queue at the border for miles.

Chertoff recently called his department’s efforts “reasonable, measured but nevertheless determined steps to getting better security.” If only they were. Any group that would like to attack us can get someone here legally, avoid controlled borders or recruit within the country. There just aren’t security gains to be gotten from inconveniencing millions of law-abiding American tourists and slowing our immensely beneficial trade with Canada, which amounted to more than $500 billion in 2006.

Security is hard, and the measures we take must cost-effectively foreclose the risks to our nation. But border checks on American citizens and national ID cards do not meet this test.

Checking American citizens’ documentation along the Canadian border will be exorbitantly expensive. At a cost of billions — directly and in foregone travel and trade — the department stands at best to briefly delay whatever harms any group may seek for our country.

The Department of Homeland Security is in thrall to the terrorism strategy. Weak actors use terrorism to goad the strong into self-injurious overreaction. Among other things, overreaction dissipates the blood and treasure of the victims.

As the United States makes war overseas, we are spending billions of dollars to shrink from international trade and travel because of the fear that we may suffer attacks that can only amount to pin-pricks on the body politic. Effective terrorism counter-strategy requires much more judicious use of our security resources.