To determine if assimilation was a successful policy before 1945, we need to determine which group we are discussing. Depending on one’s perspective, the answer would be different for different groups.
From the Native American perspective, assimilation was a disaster. Various policies have been applied to Native Americans since the 1830s. One of these policies called for assimilation. As a result of assimilation, Native American ways of living were destroyed. They had to give up their cultural characteristics and their way of life. They could no longer move about on the Great Plains and depend on the buffalo. Native Americans have lagged behind in income and education levels. The policy of assimilation was deadly for the Native Americans.
African Americans wanted to assimilate into society. However, especially in South, they weren’t allowed to do this. After Reconstruction ended, African Americans were segregated from whites. African Americans wanted to be treated the way whites were treated, but this didn’t happen because of the Jim Crow laws. Thus, assimilation didn’t work for African Americans either.
Asians and Europeans assimilated into our society. When they first came to the United States, it was difficult to learn a new culture and give up familiar ways of living. Some faced significant discrimination at first, but eventually they did assimilate into our society with fewer problems. Income and educational levels are higher for Asians and Europeans than they are for other groups. Assimilation was easier for these groups than for other groups.
Thus, you must consider the lens from which you are looking at the topic of assimilation. Different groups would have a different answer to this question.