Hijacking missions on campaign
What happens to the campaign if I hijack several missions modifying its targets, for example destroying all bridges I can, in order to prevent the enemy breaking out into my territory ?
AI ATO is managed by an occasional evaluation and generation cycle. Disrupting the ATO by manually altering missions will have no long term effect on the ATO. The AI will manage the future situation as if those past missions were flown and simply unsuccessful.
On the subject of bridges specifically the mobility of ground forces is affected, friendly and enemy. This might be out of date but last I learned single-span bridges may be repaired eventually (few days) while multi-span bridges will not. Depending on the campaign goals you may hamper or entirely prevent your own success by limiting own mobility despite the advantages of limiting enemy mobility. When in doubt consider channelizing the enemy into a narrow or wide lane of mobility which you can manage instead of completely blocking mobility. If you’re confident that bridge destruction is worth it, then it can be effective.
+1 to what Frederf wrote. Destroying a bridge here or there is OK, but a massive bridge busting campaign will, err , bust your campaign.
Instead, consider fragging interdictions along the enemy’s axes of advance. If you have A-10s in your campaign use 'em. They are seriously effective. In a recent Rolling Fire campaign the A-10 squadron killed over 2000 ground targets in the first 24 hours, stopping the Red attack cold, and Blue was launching an offensive toward Kaesong at 2100 hours on day one. Not a single bridge was targeted by Blue, though Red did knock a few out. Check this post for some info on how to do this.