结成联盟往往是为了共同防御。
英国和葡萄牙两国之间有一个长期的共同防御条约。
Britain has a long-standing treaty with Portugal about mutual defence.
在和平时期，前沿防御清楚地表明美国对于共同防御的投人。
In peacetime, forward defense also represents a clear manifestation of the U. S. commitment to the common defense.
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