How can I return my troops when I'm halfway into the attack?

This depends on the attack. Not all of them can be recalled from. To recall an APC tough you need to have completed the research (for each APC) called 'Recall Order' which is about half way down each APC research tree. Then you will see a recall option when the APC is marching. You will need to use a march recall item to complete the recall, which can be found sometimes as rewards or bought from the store / alliance store.