Is it possible to have a conditional union statement in a stored proc?

Here's an example on the northwind database. If says there's a syntax
error near the UNION statement. Looks like it doesn't like having the
BEGIN directly in front of it.

Is the only solution to create a dynamic sql string then call exec on
it?

Any help appreciated.

Tom.

CREATE PROCEDURE usp_test
(
@both int = 1
)
AS

SET NOCOUNT ON

SELECT * FROM territories WHERE regionid = 1

IF @both = 1
BEGIN

UNION

SELECT * FROM territories WHERE regionid = 2

END
GO