It's not obligatory

You can find more explanation here https://www.wordhippo.com/what-is/the-meaning-of/indonesian-word-167b6c4a4e415fdfc65024a01a1d46b38344ab1b.html

Maybe, "yang" likes to be

It can mean many things...

I think yang is more like that. The translation given does not indicate the use of yang. It translates back to Indonesian literally as -- Siapa memulai serangan ini. A more useful answer would be -- Who is it THAT initiates this attack? Literally: Who that initiates this attack? Both are correct English translations, but the longer version (Who is it that...) is a more appropriate translation (of Siapa yang...) IMO.