Dead Battery or Security system problem

[DB.Miller]

Hi,

My 2003 Infiniti M45 refused to cooperate today.
Initially I thought my Key FOB wasn't working when i depressed the unlock button and perhaps the device simply needed a new battery. I changed the battery in the Key FOB. It still didn't register with the car. I used the physical key to open up the car. I tried the Key FOB to turn the car on. Nothing. The red indicator light still flashes in the Key FOB.
I tried using the physical key to start the car. Nothing. I popped the trunk and attached a portable battery charger to the car battery terminals. When I turned it on, the lights flashed and the alarm system started gong off. I disconnected the battery charger.
I went back to the dashboard and noticed the security indicator light was flashing. I am unsure if it was when i entered the car. I tried using the Key FOB and the physical key to start the car again. Nothing.
What are the likely causes of this based on the evidence i have provided.

Thank you in advance for your contributions

[Ilya]

My guess is battery.

[Rogue One]

My guess is battery.

DITTO. Start with the simplest things first.