It looks a bit like ragweed.

I agree it does look a bit like ragweed, but ragweed's leaf terminals are much more pointed. That's the closest fit, though, I agree. And maybe it's a variant. I'm digging both up today! Neither my dog nor I need ragweed around.

This is Geranium. Bruise the leaf and smell it.

It has no smell whatsoever! But I can't believe I didn't try that test till you mentioned it. The three varieties of geranium I grow all have the distinct smell we all associate with perennial geraniums. But whatever this is, it is virtually smell-free.

I agree with tyrlych. I have something that pops up in my garden that looks like it. Really small, light pinkish flowers? I think it came with a "wildflower" seed mix I used before I knew better. It doesn't match any of the Cranesbills (Geraniums) in my Peterson's Field Guide, so I have no idea which particular species, but I'm pretty sure mine is an annual. It comes up easily, anyway.

Just came acroos this, it's what mine looks like, anyway. When I googled G. dissectum most of the flowers were darker and a little bigger, but that's not unusual I suppose. Also NOT native to US, just widely naturalized (probably from those stupid "wildflower" mixes, like I got it from). Annual or biennial.

Here is a link that might be useful: Further on down on the forum-Is this a weed?