One addition, I think space exploration can succeed just fine without global cooperation.
The US went to the moon. It was a flag & footprints mission, but we did everything we planned because there was only one country squabbling internally over every detail.
The ISS is an example of what happens when you make it international. It is non-functional, is nowhere near meeting any of its construction, budgetary, or technical targets, and makes every avid space fan groan with embarrassment because it makes everyone else think space is impossible to pull off.
Don't get me wrong, I love Russian space technology. Their Soyuz is a perfect model of a succesful, mature, and well designed technology. Their Proton boosters have amazing lift and cost. The Buran could have been great! My beef is not with the Russians, it is because the level of project management that needs to exist to pull of an international mars mission does not exist. There are people out there with the skill to wrangle a bunch of countries and make them work together to get stuff done, but those people will never be given the authority they need to do their jobs. There is a precedent that has been set that says the every time the money hits a certain point, project management suffers and politics takes charge.
I'm fine with international cooperation, but I challenge the assertion that it is absolutely necessary to pushing out the frontiers.