A more thorough explanation:

An offensive and defensive league is an agreement between countries to not only support each other when under attack but also when attacking in offensive warfare. This means that if one country in the league is attacked, the other countries will come to its aid. However, if one country in the league decides to launch an attack, the other countries are also obligated to support that attack.

For example, the Triple Entente was an offensive and defensive league formed between France, Russia, and the United Kingdom before World War I. The countries agreed to support each other if any of them were attacked by the Central Powers, which included Germany, Austria-Hungary, and Italy. However, when the war broke out, the Triple Entente also launched offensives against the Central Powers, with each country supporting the others in their attacks.

Another example is the North Atlantic Treaty Organization (NATO), which is an offensive and defensive league formed between North American and European countries. The countries in NATO have agreed to support each other if any of them are attacked, but they have also launched offensives together, such as during the Kosovo War in 1999.