A Pro-Russian activist stands near the burnt Ukrainian army tank on the outskirts of Donetsk. (Igor Kovalenko/EPA)

VLADIMIR PUTIN has responded to the international outrage over the destruction of a Malaysian airliner by his proxies in eastern Ukraine by escalating his aggression. According to U.S. officials, tanks, artillery and other heavy weapons have continued to cross from Russia to Ukraine since the passenger jet was shot down. On Wednesday, two more Ukrainian military jets were hit by anti-aircraft missiles, which Ukrainian officials said had been fired from Russia. The State Department also said Thursday that Russian artillery was firing at Ukrainian positions from across the border.

The Russian president is clearly not impressed by Western responses to the killing of 298 innocent people and the subsequent attempt by his government and its proxies to deny and cover up the crime. And why should he be? After making a statement Monday that contained no tangible response and only a vague threat that “the costs for Russia’s behavior” will increase, President Obama departed for three days of fundraising on the West Coast. The message to Mr. Putin — not to mention the Israelis, Palestinians and Iraqis fighting their own wars — was that the president was not engaged enough by the crises to set aside the purely political activity of collecting checks from donors.

In Brussels, European Union officials met Thursday to discuss potential sanctions against Russia, including new measures against the banking, energy and arms industries. But no decisions will be made before next week, and even then Moscow will likely be given a new deadline for meeting a demand that it stop supplying the Ukrainian rebels. Previous deadlines to cease weapons deliveries have passed with no significant action.

While the West temporizes, a de facto Russian army is rapidly assembling in occupied portions of eastern Ukraine. A report in the Financial Times, sourced to U.S. intelligence officials, says it includes dozens of T-64 battle tanks, Grad rocket launchers, self-propelled guns, infantry combat vehicles with automatic cannons and armored personnel carriers, in addition to anti-aircraft systems like the one that shot down the Malaysian plane. This force is commanded by Russian citizens who infiltrated Ukraine from Moscow, including a Russian secret police colonel, and made up in large part of fighters from Russia.

Incredibly, the European Union’s position — tacitly supported by Mr. Obama — is that the Ukrainian government should stop attempting to expel the invaders from its territory and instead negotiate with them about the political future of Ukraine. Fortunately, newly elected President Petro Poroshenko has not capitulated to this appeasement strategy. However, his appeals for military aid from the United States and NATO, or at least more substantial sanctions, have so far been turned aside by Mr. Obama and the Europeans.

Frustration with Mr. Obama’s weakness now extends to the top ranks of the Democratic Party. A letter released Tuesday by three Senate committee chairs — Dianne Feinstein (Calif.), Carl Levin (Mich.) and Robert Menendez (N.J.) — called on Mr. Obama to “impose immediate broad sanctions” against Russia’s defense sector, as well as broader measures against energy and financial industries, and to explore designating the rebels’ political structure as a foreign terrorist organization. While cooperation with Europe is desirable, the senators said, “the United States must not limit its own national security strategy when swift action will help fulfill our strategic objectives.”

Mr. Obama has already missed the opportunity for swift action to stop Mr. Putin’s escalation. If he does not act soon, it may be too late to save Ukraine.