Without more context, it's somewhat difficult to determine, exactly. But, here's my take: If a country is too much afraid to stand up for itself against a possibly stronger country, just to avoid war, they may find themselves under the control of that threatening country. It might be better to fight than to give in too quickly.
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pructusIs it, "Beware, lest (in your anxiety) to avoid war, you obtain a master."?No. Wrong parsing. "to avoid war" describes the kind of anxiety. It's not "lest to avoid"; it's "lest you obtain".