Sending armed troops into a foreign country for the purpose of seizing territory is an act of war, and a line not often crossed in recent decades.
It's what President Vladimir Putin has done in Ukraine — in clear violation of the United Nations Charter, the 1975 Helsinki Final Act, the 1994 Budapest Memorandum (which "guaranteed" Ukraine's territorial integrity in exchange for the surrender of its nuclear weapons), and the 1997 Ukraine-Russia Bilateral Treaty.
If the United States takes no serious actions in response, lessons will be learned — and not just by Mr. Putin.
What can an American president do? No one expects Barack Obama to put "boots on the ground." Serious economic warfare — using "banks instead of tanks," as Russian dissident Garry Kasparov has proposed — may be impossible because Europeans have grown dependent on Russia for natural gas.
The answer is not to posture. Nor, I think, is it to punish Russia directly. Instead, recognize that Mr. Putin, along with Iran's supreme leader, China's rulers and other dubious international actors regard the diminution of American power as their strategic goal, a necessary precondition for the achievement of their regional and global ambitions.
So make it clear that the weakening of America stops right here and right now. Do that by implementing policies to strengthen America. This will frustrate our adversaries and enemies, and bolster our allies. The following are four such empowering policies:
First, restore missile defense: Five years ago, President Obama canceled plans to build a Europe-based missile-defense system. Why? To please and appease Mr. Putin, who thought it possible — and unacceptable — that such a system might be used to protect Americans from Russian missiles, in contravention of the Cold War doctrine of Mutually Assured Destruction.
We should make it clear that henceforth, we intend to protect ourselves — without apology. America has the technological know-how to build a system that could prevent any intercontinental ballistic missile from any country reaching its intended victims anywhere in the world.
What we've lacked is the will to stand up to the self-proclaimed "peace activists" who prefer voluntary vulnerability.
In addition: Cancel the 2010 New START arms-control treaty, which was a great deal for Mr. Putin (no cuts of deployed warheads or strategic launchers), and a bad bargain for the United States. (We have reduced our arsenal.)
As former Sen. Jon Kyl and other leading defense experts have long urged, take steps to extend the life of America's aging ballistic nuclear warheads. Mr. Obama said he would do that. He has not.
Second, get energetic: Two years ago, Mr. Obama promised "an all-of-the-above strategy for the 21st century that develops every source of American-made energy." He has done next to nothing to fulfill that pledge, too.
Energy abundance and diversity should be our goal. That means more fracking. That means tapping petroleum on federal lands. That means ending the ban on "flexible-fuel" vehicles capable of running on a variety of liquid fuels.
That means eliminating bureaucratic barriers to entrepreneurship and competition — with investors, not politicians, attempting to pick winners. That means eliminating environmental rules that impose more costs than benefits.
A byproduct of such policies: They would create jobs and reduce poverty — because the poor spend a larger percentage of their income on energy (electricity, gasoline and heating and cooling of their homes) than do their wealthier neighbors.
Cheaper energy also would stimulate economic growth. A bigger American economy means a more powerful America.
Third, make a Group of Eight — Minus 1: The Group of Six was founded in 1975 as a forum of the world's leading industrialized democracies. When Canada joined the following year, it became the G-7. Russia was added to the club in 1998 despite the fact that it was not then — and is not now — an industrialized democracy.
On the contrary, Russia is an autocracy and relatively underdeveloped, with per-capita wealth about a third that of South Korea. What riches it possesses have not been created through invention, innovation and productivity, but through the exploitation of natural resources controlled by oligarchs.
Let's return to G-7 and, over time, transform it into an association of free-market, liberal democracies — an alternative to the United Nations, a broken institution beyond any hope of repair.
Fourth, respect the wisdom of "Si vis pacem, para bellum." That's Latin for "If you want peace, prepare for war," a doctrine dating back to Plato. Mr. Obama does not subscribe to it. Instead, he assures us that the "tide of war is receding."
However, Iran, the world's leading sponsor of terrorism, continues to spin centrifuges. Al Qaeda forces are fighting in Syria, Iraq, Yemen and North Africa. China is throwing around its growing military weight — including a 12 percent increase in military spending for 2014. Again, Mr. Putin's troops have seized Crimea, six years after having taken two big bites out of Georgia.
You don't have to be Clausewitz to see that this is the wrong moment for the United States to take another "peace dividend," to shrink the military, reduce capabilities and readiness.
The list above is by no means exhaustive. The point is to adopt policies that will make the United States stronger — economically, militarily and, by extension, diplomatically.
Nothing is more likely to cause Mr. Putin to regret his actions and think hard before repeating them elsewhere. Nothing would send a clearer message to Iran, China and other aspiring empire-builders.
"You don't just in the 21st century behave in 19th-century fashion," Secretary of State John F. Kerry said the other day. That's true in the sense that top hats and petticoats are no longer stylish.
Despotism, however, seems to be making a comeback. In consequence, the United States has 21st-century responsibilities. If we're unwilling or unable to shoulder them, no one else will.