Game Engine for a 2D RTS

OK, so I have dabbled in C++ for a considerable length of time, and have decided to make a game.
Specifically a 2D RTS. This is to be my first 'big project'.
Can anyone suggest a good (free) engine that will comply with these criteria? Also, I am planning to make this game on a low-spec laptop, so it needs to run well.
How about you use OpenGL and SFML for a 2D game? SFML is very 2D friendly, and is one of the best of its kind!
Thank you very much indeed! I'll check it out.
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