でもあった

was also

→ だ
phrase Rare #37,233
was also Used after a noun or noun phrase to add another past identity, role, or description. The でも means “also,” and あった is the past form of ある.
JA: 彼は優れた教師でもあった。
EN: He was also an excellent teacher.

彼は偉大な将軍政治家でもあった

He was a great general and statesman.

彼はかつて大学教授であり研究者でもあった

He was once a university professor and a researcher.

シュバイツァー医者であるばかりでなく音楽家でもあった

Schweitzer was not only a doctor but also a musician.

彼女は女優であるばかりでなく有名な画家でもあった

Not only was she an actress, but she was also a famous painter.