Objectively speaking, on the one hand, in modern history, the aggressor developed in science and technology, and the economy and system of the invaded country were backward and corrupt. The aggressor has brought profound disasters to the invaded country, seriously damaged the sovereignty and interests of the invaded country, and brought more heavy oppression and exploitation to the people of the invaded country. It has caused great impact and damage to the civilization, social structure, production relations and economic structure of the invaded country. On the other hand, the invaders brought advanced science and technology and advanced social ideas to the invaded country, promoted the awakening of the people of the invaded country, promoted the economic and social development of the invaded country, and made the invaded country quickly integrate into the international community. (although unequal in status)
For example, China suffered many invasions by foreign powers, which brought disasters to China, the people of China suffered tremendous triple oppression, and China lost several million square kilometers of territorial sovereignty. The powers also brought advanced social ideas such as science and technology, capitalism and socialism, feudal relations of production, the gradual disintegration of the natural economy, and the awakening of the people of China, which eventually overthrew the feudal dynasty, ended the feudal monarchy, established a bourgeois republic, and later completed the historical task of national independence and people's liberation and established a socialist republic.
For another example, when Japan was invaded by foreign powers, it did not resist, but actively learned from the West and carried out political reforms. It quickly completed the historical task of reforming the political system, production relations and social system, developed rapidly, and soon became a member of the club of great powers.
Having suffered the same aggression, China regarded the Opium War as a historical shame, and Japan thanked the American invaders for bringing them advanced ideas and advanced science and technology and promoting their modernization.
Subjectively speaking, the sovereignty and interests of the invaded country suffered huge losses, irreparable losses, and the people were oppressed and exploited by the aggressors, even massacred on a large scale and almost died. Therefore, opposing aggression is meritorious. Although the economy, science and technology and politics of the invaded country are relatively backward, the history is progressive. It is only a matter of time before feudalism is replaced by capitalism and capitalism by socialism.