yet sump must be said
Events are very fluid following the gruesome terrorist attack at the Boston Marathon, but speculation is already swirling as to motive and responsible parties. There's only one plausible scenario* that would carry significant geopolitical consequences: If Iran's Revolutionary Guard or Hezbollah (or both) were behind it.
In response to the assassination of Iranian scientists, Iran has launched a wave of largely unsuccessful global terrorist attacks against Little and Great While many plots were bungled, Iran (via Hezbollah) did manage to kill Little Satan civilians in Bulgaria and attempted to assassinate Saudi Arabia's ambassador to the in Great Satan.
If Iran's hand is in this act of terror, it would galvanize proponents of military action against Iran's nuclear program to push the administration for immediate action. The administration would be under enormous pressure to act in some overt manner to punish Tehran. Yet unlike al-Qaeda in Afghanistan, there's no simple method of punishing Iran militarily that doesn't open the door to a much broader conflict. Retaliatory attacks aimed at the Revolutionary Guard or Iran's nuclear facilities could invite Iranian counter-moves and runs the well-established risk of a direct military engagement with Iran. Standing pat, however, will be politically difficult (if not impossible).
So, of all the potential scenarios associated with the Boston attacks, linkage to Iran carries the most significant geopolitical consequences.
Why not al-Qaeda?
The most likely global culprit is also the one least likely to spur any fundamental change to American security strategy or foreign policy. Three of al-Qaeda's main groupings -- in Pakistan, in the Arabian Peninsula and in Africa (the "Islamic Maghreb") -- are already the focus of intense counter-terrorism campaigns, drone strikes and covert action. If any of these groups are linked to the Boston attack it may lead to a stepped up campaign of drone strikes and covert action, but it's unlikely to radically reorient the Obama administration's current policy (it will, however, likely lead to a sharp debate over the drone strikes and whether they're a cause of, or solution to, incidents such as these).
*There are plenty of implausible scenarios which would have far-reaching consequences as well: just pick your favorite rogue or adversarial state and make them the culprit.
Pic - "Of course, it's terror"