As has been mentioned before, yes it's good practise to try merging under strength units in the absence of reinforcements, however there are three very important requirements to meet before this can take place.
Time, Opportunity, and Fuel.
Without all three of these factors being available in plenty, it's just not possible.
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If it moves, shoot it, if not push it, if it still doesn't move, use explosives.
Nothing happens in isolation - it's called "the butterfly effect"
Mors ante pudorem
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