“A shot across the bow” has been a much-used metaphor of late, referring to the proposed strike against the Assad regime in Syria for its use of chemical weapons.

But what does that phrase mean, exactly? In the days before radio communications it was a warning that meant, simply, “stop or I’ll sink you.” It gave the other ship an opportunity to stand to before being attacked.

But the warning, necessarily, implied the possibility of further hostile action. After all, if you tell someone to “stop or I’ll sink you,” and they don’t stop, the next move is to hit them directly. If they still don’t stop, then you carry out the threat and sink them. In other words, a shot across the bow is the first step on a clearly determined ladder of escalation. It is not an end in itself.

President Obama has been desperately trying to convince a skeptical Congress, and an even more skeptical public, that this shot across the bow will be a one-off with no further military action afterwards. He has emphasized that this would be a surgical, limited strike with very limited effect on Assad’s military assets (especially as Assad has now had several weeks to get them out of harm’s way). Obama has ignored the possibility that Assad will ignore the warning.

As far as I know, only Senator Susan Collins (R-Maine) has asked the question, what happens if, after a U.S. attack, Assad uses chemical weapons again? As she says, if we strike again, that is the very definition of “further involvement.” If we don’t strike again, then the United States is exposed as the maker of empty threats and can be safely ignored. No Great Power can allow itself to be so exposed.

The president will be interviewed tomorrow by no fewer than six TV networks (ABC, NBC, CBS, PBS, CNN, and Fox). I hope at least one of the interviewers will ask him—and insist on a clear answer to—the question, what happens if we hit Syria and they go on using chemical weapons?