As has been a recurring theme since the start of the crisis in Ukraine, United Nations Ambassador Samantha Power could not have been clearer in her August 8 address to the United Nations. Speaking about a proposed “humanitarian aid” convoy which Russia indicated would penetrate Ukraine’s border, Power warned Russia that the United States would regard this as a very specific violation of international norms and obligations.

“[A]ny further unilateral intervention by Russia into Ukrainian territory – including one under the guise of providing humanitarian aid – would be completely unacceptable and deeply alarming,” Power said at a meeting of the United Nations Security Council. “And it would be viewed as an invasion of Ukraine.”

She did not throw out that word “invasion” casually. This statement was vetted by the administration. It was designed to establish the gravity and the implied consequences associated with carrying out what the United States considered an assault on sovereign Ukrainian territory.

On August 22, however, that “humanitarian aid” convoy did invade Ukraine. The raid was accompanied by artillery fire from Russian units both inside Ukraine and from across the Russian border. The word “invasion” was never heard again.

On August 27 and 28, Russian intervention in Ukraine became overt. American officials warned that Russian regular forces, not proxies or pro-Russian rebels, were engaged in combat operations inside Ukraine. Senior NATO officials said that more than 1,000 Russian troops were fighting alongside pro-Moscow rebels, but the official stopped short of calling it an invasion, preferring instead to call it an “incursion.”

Power, too, opted to avoid using the “I” word, as did her boss, President Barack Obama. This caught the attention of The New York Times which noted in its headline that Obama steadfastly avoided calling Russia’s “advance an invasion.” Even America’s overseas allies got the memo.

Why does this matter? Well, words have meaning. Particularly when they relate to legal affairs or international accords, and this is precisely what is at stake. Specifically, a 1994 diplomatic memorandum which Russia and Western powers signed that guaranteed Ukraine’s territorial integrity under the condition that they surrender the nuclear weapons stockpiles bequeathed them following the demise of the Soviet Union.

“Bill Clinton, John Major, Boris Yeltsin and Leonid Kuchma – the then-rulers of the USA, UK, Russia and Ukraine – agreed to the The Budapest Memorandum as part of the denuclearization of former Soviet republics after the dissolution of the Soviet Union,” The Daily Mail reported in February. “Technically it means that if Russia has invaded Ukraine then it would be difficult for the US and Britain to avoid going to war.”

The memorandum is toothless, there is no legal means of enforcing it, and it was in fact violated in late February when Russia invaded and later annexed the Crimean Peninsula. Nevertheless, diplomats get nervous when governments start ignoring their international obligations. It makes other treaties and memoranda that much more difficult to enforce, fosters global instability, and creates the conditions for future “incursions” by other major powers with territorial ambitions or designs on their neighbors.

Some suggest that this diplomatic dance is foolish. The United States is not going to go to war with Russia, much less over a non-NATO country where few U.S. interests are immediately at risk. Why not be honest about that? A domestic and international audience would appreciate the clarity, this argument goes. The response is that taking tools, any tools, off the table during a negotiation – particularly a crisis negotiation – limits freedom of action.

One never takes a tool off the table during a negotiation without reciprocity from the negotiating partner. To do otherwise is to set a bad precedent, one which a smart negotiating partner will make you repeat.

President Obama insisted that the sanctions regime he has imposed on Russia is working, that he will not approve a military solution to the crisis in Ukraine, and that providing lethal aid to Kiev’s forces is not under immediate consideration. What, then, is on the table? Our options are increasingly limited while Moscow’s freedom to escalate or de-escalate as he sees fit remains robust.

In 2012, Obama set a “red line” which would necessitate action against Syria if and when they used chemical weapons. He walked away from that threat the following year when it became clear it would be politically disastrous for him to follow through with it. At that moment, he took an option off the table without reciprocity from his negotiating partner – who, at the time, just happened to be Moscow negotiating on behalf of its client in Damascus.

Vladimir Putin has made Obama repeat his mistake.