a drastic war

I don't think that really works. You can use drastic obliquely, by saying "a drastic measure" or "a drastic way."

Declaring war is a drastic way to resolve a border dispute.

However, I did find some uses of "drastic war."

Charles Royster, author of a 1991 book about the American Civil war used it more than once
>Americans did not invent new methods of drastic war during the Civil War so much as they made real a version of conflict many of them had talked about from the start.

and
>Drastic war brought emancipation and some of the other results that slavery's opponents sought.

But otherwise, it was mostly things like
drastic war taxes
drastic war laws
drastic war experiences
drastic war policies

"Drastic wars," plural, is more common.

Europe had recovered from drastic wars and peace settlements before.
[Armageddon] is the great epic, in which USA and Europe will be defeated, and it is one of the most rigorous and drastic wars.
Despite the various types of drastic wars and events, our beloved Beirut remains the Queen of all Capitals.
And, closer to your question:
Bosnia's appearance after drastic wars seems to be constituted worse than a pre-war successful diplomacy would have designed it.

"Drastic" collocates more naturally with things that happen once or in a short period of time, rather than over the course of years: a drastic decision+, +a drastic mistake+, +a drastic measure+, +a drastic gambit+, +a drastic attack.

As #1 demonstrates, "drastic" can be used with "war," but it's probably not the best choice.