Yap.Town

の義務だ

is one's duty

→ の
phrase Uncommon #14,811
is one's duty Used to state that something is someone's duty or obligation. The part before の indicates whose duty it is, and だ means “is.”
JA: 困っている人を助けるのは、私たちの義務だ。
EN: It is our duty to help people in need.

助けるのが我々の義務だ。

It is our duty to help.

法律に従うのは我々の義務だ。

It is our duty to obey the law.

俺の義務だ

It's my duty.

— 灼熱の魂 (2010)

法に従うのは我々の義務だ。

It is our duty to obey the law.

税金を払うのは私達の義務だと考える。

I believe it is our duty to pay taxes.

法律に従うのはみんなの義務だ。

Everyone has a duty to obey the law.

税金を払うのは働くものすべての義務だ。

Everyone who works has a duty to pay taxes.

法律に従うことは皆の義務だ。

Everyone has a duty to obey the law.