The option to declare war in later versions is rather redundant.

Set the destination of a warship to a ship of the one you want to attack. When they get there you will have the option to attack (and thus, declare war).

Set the destination of a troop transport to a planet you want to invade. When they get there you will have the option to attack (and thus declare war).

Simple.

Declaring war in a diplo screen is simply more mouse clicks and serves no usefull purpose. It is just another step before you actually attack a target. And if you wait for the next turn after you declare war in a diplo screen to attack, well, you just lost the advantage of surprise (and a turn-full of destroyed ships and conquered worlds).

The AI usually declares war, then waits until the next turn to actually attack. This can be to your advantage, if you have warships and transports close enough to their own to do some damage on that same turn.