I didn't say there'd be no ways of defending ourselves. Please highlight where I had suggested this.
What I did suggest is that encouraging an arms race in outer space - ignoring the failure to adhere to the outer space treaty - is not the most productive way to bring about peace.
Lets pretend that rather than just allowing it to be given (which is the easiest option for them, and to paraphrase Neitzsche: To predict the behavior of ordinary people in advance, you only have to assume that they will always try to escape a disagreeable situation with the smallest possible expenditure of effort.) the Earth, or facets therof, decides to attempt to take for themselves our resources or facilities - as opposed to simply getting their own from the rediculously abundant resources available in space.
It's literally an uphill struggle all the way to us - A lot of effort. And what do you think will really be waiting on the other end for them? Hope of goodwill or an intelligent design of countermeasures ready to and capable of mitigating any eventually? What will defend the nation is not the mind itself, raw and mushy, but instead the product of it's effective use.
Almost any type of ballistic projectile is possible to be countered as or shortly thereafter leaving the atmosphere. By numerous methods. When applicable, particle and energy based weapons shall be mitigated equally trivially. Then to consider facilities (if adhering to sanity, we would have mulitple distributed facilities, this gives rise to nearby support and distributed responses, and further represent the most difficult thing take out) large enough to serve as realistical habitations for masses of citizens would generate an insane amount of debris - much of it large, with potential to achive orbital deacy and survive re-entry enough to cause a lot of widespread surface damages - the rest simply taking out Earth's satellite networks. This action (attack from surface via weapons) is unlikely to be persued. Any attempts rendered futile with ease, and even if it was to magically somehow occur and actually work -=- The end results are not going to be good for anyone.
This leaves, then, classical invasion.
There's dozens of non lethal ways to prevent a craft from obtaining a range that would leave us unable to have effective countermeasures to any weaponry it could be carrying - and considering the weight issues with most armor plating, and them having to lift that from Earth, it's likely to be able to incredibly easily utilse lethal means to prevent docking attempts, should we require, with the most primitive of methods let alone anything "mispurposed" (say, a 10-tonne cargo pod in a centrafugal launcher. Instead of throwing those ten tonnes of raw materials to a processing plant/factory, set collision trajectory. Any tool is a weapon, if you hold it right).
I'd not entirely trick the bear. First up, intelligence suggests should I be concerned about bears, it may be more prudent to not intentionally place myself into situations that are likely to occur bears. It's not an overly good analogy, being both a lone agent and a force of nature and what with you effectively treading into the bear's territory and as defined in the outer space treaty, space is no territory, You are going to them... not them comming to us... But should one be planning (which, suggests use of intelligence) to go somewhere that would occur bears I'd possibly make the effort to learn quite a lot about bears. Specifically, how they operate as both individuals and social groups, their precise physiology(it's not just about how hard you hit, where you hit can mean so much more) including various biological mechanisms like the use of scent/pheromones, and anything else relevent to raise tactical advantages and understand as much what the bear can't do as much as what it can. It should be posssible to engineer circumstances so should you occur a bear, there is no requirement for defense.. I'm also to understand they are intelligent... but should that not be applicable, you can exploit various deficits in the bears basic design - and if you "productively" layer these in the right sequences, you can predict and control the bears response making everything so much easier.
Yes, if the intent is purely execution, having a gun is possibly an easy way to achive. But it's certainly not the only way to achieve. And you seriously don't want to be putting all your eggs into that basket.