Yay, an opportunity to drop one of my favorite historical quotes!
Whatever happens, we have got
The Maxim gun, and they have not.
The Maxim gun was an early machine gun, and one of the primary reasons that Britain was able to do exactly what you state on the scales that it did. Though the psychological effects of the weapon are often credited equally or moreso than the weapon's actual lethal power, it makes no difference in the end.
We live in a different world today than we did in the age of Imperialism, but in some ways the groundwork that was laid and the ideas and goals are still in place. We ship capital and labor overseas rather than challenging a nation's political soverignty, but in effect we still extend our own power and gather resources. We've just gotten more diplomatic about our imperial ambitions.
Likewise, we live in a world where running in with a machine gun is just going to result in answering to the opponent's machine guns. The Maxim gun strategy is dependent on two conditions: that we have the Maxim gun, and that they do not. The strategy breaks down when both sides have a Maxim gun as badly as when neither side does. In fact, we've so thoroughly advanced the art of physical destruction that we can completely destroy any nation on Earth that we want to--or all of them, if we desired--with a sufficient number of missiles that can be launched and make contact in minutes. And the most compelling reason not to is that they can do the same to us (at least, if the two countries happen to be the US and Russia, who own the vast majority of known nuclear weapons).
What we require, then, is to find a method of conquest that our small nation has but the large nation can't match. Which would mean that the small nation needs to be a highly tech-savvy nation. Zero day attacks against the larger nation's infrastructure may be effective, especially with careful planning. Playing your cards just right, a cyberattack can allow a nation with high technological prowess to attack another nation's systems in such a way that their involvement can not be proven and thus retaliation is unsuitable. In fact, this has been done before. Choosing the target systems carefully can allow the attacker to do anything from destroying powerful weapons to disrupting logistics, leaving them wide open to invasion.