Usually the best defence is offense; however, for interplanetary war, defence has an overwhelming advantage--efficient use of energy and overwhelming resources. Propellant availability and usage makes waging war even within a solar system extremely difficult and between stellar systems almost impossible. The aggressor would we better off "terraforming" rather than conquering.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?