Ammonium nitrate and a accelerant. This combination was the explosive used in the worst case of Domestic Terrorism in US history. It doesn't take much to make a big bomb, and I can get both ingredients locally in the form of Fertilizer and Diesel Fuel. Nasty Girls, that's where. We have a major army base just down the road. Desert Warfare Training center specifically. They have all the shit we would need. Concertina wire is cheap and readily available. I can order the shit on ebay for -blam!-'s sake. They are not determined specifically to attack my college. It's more a case of an entrenched enemy position threatening their flank on their advance into the interior of the country. Sure, they could siege, but a siege takes time and resources that could be diverted to the advance. No army will let a fortified position sit on their flank unmolested. A break out from that position into the rear of their army would be devastating to their squishy supply lines and other logistics.