the most efficient way to detect colision between objects in a large scene (Strategy Game)

IMHO, you should approximate all your objects with bounding circles on a 2D plane (ground). I think this is what every one do in their RTS games (like StarCraft :) ) where there are a lot of small units.

Collision tests between 2D circles are very fast. It looks like this (pseudocode!!!):

    bool collided = ( Distance( circle1.position, circle2.position ) <= 
(circle1.radius + circle2.radius) );

NOTE: Finding distance usually requires Sqrt() which is slow. So it is a good idea to compare squared distance and squared sum of radiuses.

Then you will need some optimizations because you won't like to do tests for each pair of your 50 moving units every frame (I guess it is something like 2,450 tests). The purpose of optimization is to find pairs which most likely collide or to reject those which definitely won't collide.

If you have some sort of space partitioning structures like Quad tree in your game, then it can be handy.

Very simple approach (I invented it just now :) ) is the following:

Let's assume that RMax is a maximal radius of the bounding circle of your in-game unit. Then you can represent your level as a grid of square cells with a cell size of RMax. When your units move you easily define in which cells their centers lie. And knowing that you can test only those pairs of objects which lie in adjacent squares. That's it!