I.S.A definitely, trying to take what's not theirs, the helghast just want what they would if they were on helghan, a trade agreement, payment for passing trade, no different than any toll bridge really, oh but they will suffer for it, revenge is a dish best served cold..
I'm a HGH through and through, but if you read the storyline in the "info" tab the HGH are painted as the bad guys. Then again that version of history was probably written by the UCA after they won the first Helghan war, so who knows what really happened. All I know is that the HGH started taxing all through space traffic so they made huge amounts of $$$ but the UCA was having a hard time on their other colonies, so the HGH offered to buy their system outright and could pay it all upfront.
Then they upped their taxes even more and built a fleet of ships, mind you every ship had to pass through their system on their way back to earth. Eventually the UCA couldn't take it anymore and sent its fleet in to secure a safe, tax free rout for their ships, the HGH resisted, war broke out, they blasted Helghan and Vekta from orbit, took over Vekta and put pro-UCA forces in power. The HGH on Vekta all decided to leave in a mass-exodus. There was a power vacume on Helghan which was eventually filled by the one... the only... Visari...
If you didn't play Killzone 1 then you aren't allowed to vote, it's insanely biased (HGH attacked ISA homeworld of Vekta cause they had some sand in their **bleep**'s cause when they left the first colony they got **bleep**ed up by the air and **bleep**, so after some time they realized "oh snap its growing our dicks we can kick the **bleep**ing **bleep** out of those tree-having **bleep**s")