This Missing scene from the film explains the massive plot hole

Rating: 1/10 (17 February 2011)

The alien General greets his sergeant as he enters his chamber. 'Hello Splenkins thanks for coming. I wanted to go over the invasion plans again.'
Splenkins looks perplexed. 'We're still going ahead with the invasion? It's just you know, there is so much water down there and you know how we don't get on with the stuff.'
The General waves dismissingly.'Detail Splenkins, details.'
'With all respect sir there is loads of it down there, I mean whole great oceans of it. Then you have the streams, ponds, rivers. Schmek! It even falls from the sky.'
General looks unimpressed.
Splenkins smiles suddenly. 'Well I suppose we won't be down there long, a few quick blasts from the Old Heat ray and we'll be in the White House and back in time for dinner.'
'I'm afraid we're not going with the Heat Rays on this one.' The General shrugs. 'Budget cuts and all.'
'Oh. Well just laser guns and De-atomisers then.'
'Nope. I'm afraid no lasers or De-atomisers either. Actually no fire arms of any sort are available'
Splenkins looks down at his feet, his forehead furrowed. 'Oh well I'll get me a big lump of space metal and beat those humans until'
The General shakes his head apologetically.
Splenkins shoulders slump. 'Well as soon as I land I'll pick up the biggest stick'
'I'd rather you didn't'
'What?' asks Splenkins bemused.
'There's no need to make the place look untidy'
Splenkins is quiet for a moment, then laughs out loud. 'Why am I worrying? All this talk of water and weapons. What do I care? I'll be tucked up in my Super Heavy Atomic-Powered Battle Armour. 'I'll crush their puny tanks with my bare hands.' He gestures pushing his fist into his palm.
'Oh yes about the Atomic-Powered Battle Armour.'
'You're kidding me?'
The General shrugs. 'I wish I were. There was some administration errors and someone packed extra underwear instead of battle armour. Kind of funny if you think about it. But I'm afraid Splenkins on this occasion you're going to have to take on the might of the human menace... in your pants.'
'Sir by any chance has this got anything to do with me sleeping with your sister?'
The General grins. 'What do you think soldier? Now go get them! Don't forget to pick up your army issue Y-fronts on the way out, and beware little girls baring glasses of water.'
Splenkins trudges towards the teleporter saying some very nasty things in Martian.