Yes it's an op-ed, but don't fantasize that it's part of some sort of balanced wide-ranging array of varied opinions. The Washington Post wouldn't print a column advocating peace to save its life — as such an act just might help to do. And you can imagine the response if the headline had been: "Racism is probably our best option," or "Rape is probably our best option," or "Child abuse is probably our best option." Nobody would object: "But they've probably had lots of columns opposing child abuse. Surely they can have one in favor, or do you want to shut down debate?" No, some things are rightly considered beyond the range of acceptability. War, in Washington, is not one of them.
Now, war propaganda is illegal under the International Covenant on Civic and Political Rights. War itself is illegal under the Kellogg-Briand Pact and the United Nations Charter. But the Washington Post isn't one to worry about legal niceties.