The paramount lesson from the Russian invasion of Ukraine is that repeated threats of economic sanctions didn’t deter Vladimir Putin from launching an all-out invasion. This offers a warning for Taiwan, the U.S. and their allies as threats from China loom.

The long history of sanctions, embargoes and economic blockades strongly suggests they are difficult to enforce, entail significant costs to the nations imposing them, and trigger market forces that eventually override them. Benefits flow to countries that don’t enforce the sanctions. The enforcement challenges grow significantly if the economy of the targeted nation is large, and as the size of the target country increases, the deterrent effect of threatening sanctions loses credibility.

Since the Chinese economy—one-sixth of the world’s economy—is 10 times as large as the Russian economy, effective sanctions would be virtually impossible to enforce. Relying on threatened sanctions to deter a Chinese attack on Taiwan could therefore entice aggression that could pull the U.S. into a war with China, an event that would alter the course of world history.

Thankfully, there is a far more effective deterrent. Taiwan is an island roughly 100 miles off the coast of mainland China. Unlike Ukraine, a large land army can’t be massed along its border. But because it is an island, supplies also can’t be delivered to an adjacent neighbor and clandestinely driven across the border. Any supplies delivered after an attack would have to be flown in or delivered by ship, putting the supplier directly in harm’s way. Supplying Taiwan on anything like the scale we have supplied Ukraine during a Chinese attack would be a logistical nightmare

When China was an economic basket case, 100 miles of ocean was more than enough deterrent. But with China now an economic and military powerhouse, Taiwan’s lack of preparedness is increasingly dangerous.

Taiwan’s economy is two-thirds larger than Israel’s, but Taiwan spends almost two-thirds less as a percentage of gross domestic product on defense. U.S. support can’t be allowed to abet Taiwan’s neglect of its own defense. As Machiavelli observed, “nothing is so weak and unsustainable as a reputation for power which is not based on one’s own strength.”

The good news is that modern technology makes it relatively easy for Taiwan to afford weapons that would make the cost of invasion exceed any reasonable benefit. Ukraine’s valiant resistance has shown how highly motivated defenders with high-tech weapons can scramble the calculus of military power. Like David’s smooth stone that slew Goliath, two Ukrainian Neptune missiles sank the flagship of the Russian navy in the Black Sea. With 400 U.S. Harpoon missiles, costing only 0.3% of its GDP, Taiwan could imperil any Chinese warship in the Taiwan Strait. Modern sea mines are even less expensive, and Turkish Bayraktar drones, which have been so effective in Ukraine, cost less than $2 million each. Two hundred fifty million dollars would buy 5,000 Switchblade drones, which could devastate landing craft, armored vehicles, and small assault ships.

Taiwan already has two Patriot missile battalions and for $3 billion could double its air and missile defense. Stinger missiles, used to great effect in Ukraine, cost only $400 million for 1,000 missiles. Taiwan will have more than 200 F-16 fighter jets by 2026, including almost 70 of the newest Block 70 aircraft. With additional F-16s and other aircraft being retired from the U.S. Air Force, more aircraft could be made available at their depreciated value.

If the U.S. and its allies are willing to accelerate the sale of these and other force-multiplier weapons at cost, Taiwan could totally upgrade and harden its defenses by simply raising its defense budget from 2% to 3% of GDP. At that level, Taiwan could fund all these weapon purchases over a five-year period. Sustaining its defense outlays at 3% of GDP would allow Taiwan to continue modernizing its defenses while spending at a level roughly equal to Israel’s defense expenditures in real dollar terms.

With these investments, Taiwan should focus heavily on training for new weapons systems. It should also consider transforming its army from the current conscript system into a smaller voluntary force that would better accommodate a defense system based on the power of modern technology. Citizens who would have otherwise been drafted could be trained in high-end weaponry and kept in reserve or home-guard forces that could be activated in emergencies.

The primary objective of the U.S., its allies and Taiwan isn’t to repel a Chinese attack but to prevent it from ever occurring. Effective deterrence is the key to national security.

Any wolf has the ability to kill a gentle porcupine. And yet such an attack virtually never occurs in nature. The defense of the porcupine’s quills, which can rip through the predator’s mouth and throat, is the deterrent that protects the small creature in the violent woods. Through the force-multiplying miracle of modern weapons, we can help make Taiwan a porcupine and deter aggression that could have profoundly negative consequences for Taiwan, China and the world.

Mr. Gramm is a former chairman of the Senate Banking Committee and a nonresident senior fellow at the American Enterprise Institute. Mr. Wicker, a U.S. senator from Mississippi, is in line to become the chairman of the Armed Services Committee if Republicans control the Senate next year.

This op-ed originally appeared in the Wall Street Journal. Read it here.