A mutual protection pact (MPP) or defensive pact (DP) is a diplomatic agreement in which participant states pledge themselves to assist each other militarily against an aggressor. In simple terms, if a nation is attacked, all its pact brothers/sisters will declare war on the attacker. In the exceptionally rare instance where a country pledges military support to another without expecting reciprocal defence, the relation is known as a guarantee of sovereignty.
Mutual protection pacts are the most common form of strategic alliance. Their primary intent is to discourage invasion through threat of international retaliation. The exact grounds for intervention can vary, and in lieu of formal treaty texts have often led to disputes over what constitutes a valid scope of interpretation: a loose reading may demand the pact's invocation every time a country is attacked (the Great War approach); a stricter reading may render the treaty null and void the moment a signatory engages in an offensive war.